Trivia: When Frenchy and Doody arrive at the National Dance Off, Frenchy asks Doody how does she look, to which Doody replies, "Like a beautiful blonde pineapple." Barry Pearl (Doody) ad-libbed this line.
Trivia: The rest of the cast did not see Olivia Newton-John in her greaser girl outfit until she walked into the scene at the Graduation Carnival. The stunned reactions of the T-Birds are real.
Trivia: Leo's name is never spoken in the movie. It is only mentioned in the end credits.
Trivia: Jeff Conaway (Kenicke) and Olivia Newton-John (Sandy) were brother-in-law and sister-in-law in the early 1980s. Conaway was married to Olivia's sister Rona.
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