Treasure hunt clues!

On Friday I'm running a treasure hunt for my peer support kids. They're a little older than the usual, 14-15 so the clues need to be reasonably hard and hopefully involve teamwork and co-operation. The hunt will hopefully last about 45 minutes. All types of things can be used, riddles, codes, maps etc. The hunt needs to stay within school grounds (as well as classrooms, corridors there is a garden, various playing fields and netball courts) and also out of classrooms but we can use the library.

I've already got a few ideas but I need more!!!! Does anyone have any ideas for some good clues? Thanks so much!!

i think you would have to know the place pretty good to come up with some clue. Like we did a hunt and the school mascot was a Eagle and we made a clue that said you'll find your next stop where wings are streached. took the kids awhile to figure it out and we all thought we gave them a easy clue. anyway good luck with your hunt hope it goes well sorry i was not much help