There are some shows where the male and female leads are not romantically involved with each other.
Kind of depends on the kids and what types and vocal ranges they are.
* Anything Goes comes to mind. Reno and Billy are the leads but he ends up with Hope Harcourt and she ends up with Lord Evelyn.
* Joseph & the Amazing Technicolor Dreamcoat - you have The Narrator and Joseph. Brother and Sister might be a great relationship for those two characters actually.
* Also Cabaret - Sally Bowles and the M.C. are both fantastic scene stealing leads and not a couple.
* Into The Woods - The Witch and The Baker
* Rent - Mark and Mimi are not romantically linked (there is a high school version of this show)
* Peter Pan - Peter (if cast as a girl) and Captain Hook, obviously.
* Hello Dolly - Dolly Levi and Cornelius Hackle also not a couple.