Danny: Uh, I'm not very hungry; just gimme a double Polar Burger wit' everything and a cherry soda wit' chocolate ice cream.
Principal McGee: We have pictures of you so-called mooners. And just because the pictures aren't of your faces doesn't mean we can't identify you. At this very moment those pictures are on their way to Washington where the FBI has experts in this type of identification. If you turn yourselves in now, you may escape a Federal charge.
Frenchy: Doody, how do I look?
Doody: Like a beautiful blonde pineapple!
Danny: You can't just walk out of a drive-in.
Rizzo: Ok, so what do you guys think this is a gang bang?
Sonny: Yeah, you wish.
Cha Cha: They call me Cha Cha because I'm the best dancer at St. Bernadette's.
Frenchy: With the worst reputation.
Frenchy: Beauty-school sure wasn't as I thought it would be.
Vi: Nothing ever is.
Vince: It doesn't matter if you win or lose, it's what you do with your dancin' shoes.
Jan: You mean you're dropping out?
Frenchy: I don't look at it as dropping out! I look at it as a very strategic career move.
Danny: You know, if we fix up this car, it could be make-out city, you know that.
Sonny: Right, the chick is gonna have to put out before she even gets in.
Jan: Don't let me near the refreshment stand! Take my money away from me! I'm not hungry! I'M NOT HUNGRY!
Frenchy: I wish I had a guardian angel to tell me what to do. You know, like Debbie Reynolds had in "Tammy." What do you think?
Vi: If you find him, give him my phone number.
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