Continuity mistake: In the Greased Lightning dance scene, look just in front of the last white desk on the right side during the dream car segment. One of the dancers knocks a tyre off a stand and it falls in front of the desk. A shot or two later, the tyre is back up on the stand. (00:40:05)
Audio problem: When Sonny says "Shut up!" to Putzie outside Kenickie's car, his mouth doesn't move.
Continuity mistake: When Principal McGee does her announcement to all the students at the beginning of the film, you can see a flag of the United States at the far end of her desk, but when we see a close-up of the Principal, the flag is now right in front of her.
Continuity mistake: In the scene where the whole gang meets at the diner, the waitress brings out Danny and Sandy's food, and you can see the drinks they have ordered are a dark pink colour. In the next shot that they appear, the drinks are a lot lighter shade, almost pale pink.
Continuity mistake: When Marty and Rizzo walk past the TV truck, Marty says "Rizz!" and they stop and look at Kenickie, whose head is tilted to the right, but in the next scene his head has moved to the left.
Continuity mistake: When the three T-Birds go to moon at the TV camera during the performance of "Blue Moon" at the National Dance Off, you can see there are two red balloons in front of Doody, but in the next shot the balloons have disappeared.
Continuity mistake: Just before Kenickie runs up behind Patty Simcox and lifts up her dress she standing at the back of the hall clapping her hands, she is not in the dance contest.
Visible crew/equipment: When Sonny snaps Tom's suspenders as he enters the gym, you can see a microphone in the top right corner.
Other mistake: At the dance, students are still arriving as the band starts to set up. A moment later, the MC announces over the mic that Johnny Casino and the Gamblers will get things warmed up, and then music starts, which is not possible with the band just setting up.
Continuity mistake: When Sandy is asking Rizzo if she's going to Thunder Road, her hair is curled under. When she's watching the race and then talking to Frenchy after Danny wins, her hair is flipped outwards.
Continuity mistake: After the Song "You're the one that I want" when Danny is doing the test your strength game you can see Frenchy running from the Ferris Wheel a second time when Rizzo and Kenickie start kissing again, when that scene happens just before the song. (01:44:50)
Continuity mistake: In the pre-credits sequence, when Danny and Sandy on walking on the beach, Danny is on the left and Sandy is on the right. However, in the next scene, when they kiss, they have suddenly changed positions. (00:00:40)
Continuity mistake: During the basketball lesson with the team and Danny, Coach Calhoun is holding the ball and tells him, "You have to dribble." At this point you can see that Danny is very close to the coach, but in the next shot he is further away from him.
Other mistake: In the opening animated sequence, it shows Rizzo taking off a top and her neck stretching. She looks in the mirror. As she is facing the mirror you can see that even with the elongated neck you still should be able to see her face in the mirror, but the elongated reflection just shows the neck looking much longer than it really should. Also, as the animation goes on you see Riz turning sideways but the reflection in the mirror does not match. She is sideways while her boobs in the reflection are facing the other direction. Then when she puts the black blouse on her back is to the mirror but we see her front in the mirror.
Continuity mistake: During the dance contest, when Kenickie and Cha-Cha are dancing around like crazy (when she puts her foot up on his shoulder), you can see Danny and Sandy in the background, standing on the sidelines. Given that they're the last couple on the floor at the end, they should still be out dancing at this point.
Continuity mistake: When Sonny enters the sports hall for the National Dance Off, he twangs Chisum's braces and walks towards the bandstand. Chisum is well away from the doorway at this point. However, in the next scene, Chisum is now standing near the doorway and Sonny is walking away from the bandstand.
Continuity mistake: Widescreen version: When Vince Fontaine introduces Johnny Casino & The Gamblers at the National Bandstand, Blanche is seen walking off the stage. In the next scene, she is on the stage, dancing with the band.
Continuity mistake: Once during the dancing scene you can see Vince Fontaine standing on Marty's left side, and then suddenly on her right. (01:10:16)
Continuity mistake: At the dance when they do that partner dance down the middle of the gym, as the lines form Kenickie and Cha-Cha start dancing down it, then the next shot its another couple first down it, and then a couple of shots later Kenickie and Cha-Cha start going down the lines again.
Continuity mistake: When Danny is running on the athletics track, he falls over one of the hurdles. He then picks him up and Sandy goes over to see if he is okay. You can see that Danny is walking towards to the field section of the track, but in the next shot he is walking away from Sandy.
Answer: Well, Danny was out of town, at the beach. It's quite possible that the rest of T-Birds didn't live close enough to each other to see each other during the summer. Also, they may have all had jobs, or been travelling with their families. It's also possible that they DID see each other, although maybe not as "T-Birds". When they saw each other on the first day of school, though, they were Seniors, and they were ready and excited to be T-Bird seniors.
BGraz
Another possible reason is that not all of them were allowed to see friends during the summer. I had a couple of classmates whose parents were strict about them focusing on schoolwork and/or getting a job. No phone calls or visiting friends on the weekends or during summer: "You can see your friends at school."
Do you really think any of the T-Birds had strict parents?
People quite often rebel against strict parents.
When they asked Kenickie where he was, he said "working, which is more than I can say for any of youse kids" suggesting that the 3 stooges (pun intended for their stupid routine that prompts Danny to tell them to "be cool") didn't work all summer. Also, Sonny needed to borrow money in the dinner until he could get his allowance.
In regards to not living close enough to each other, it is worth mentioning that having access to a vehicle was much less common compared to nowadays.
KeyZOid