I was a flute/piccolo player as well in band! And, the brass section… hahaha!
You can range from switching their valves around, or during practices… sneak up behind them as they are about to play so their tuning slides fall out! So, that way they can sound ridiculous and out of tune instead of the flute section for once! :D
Or if they leave their instruments around, steal their mouth pieces! :D And give them to random people in the band! :D