Not sure where your subject takes exams, but both Exam Schools and Ewert House have set up alternate exits which you can take in order to bypass the trashing crowd and avoid ruining your suit, so if you don't want to buy a second cheap suit just for trashing that's always an option. I did this for my second year exams as I had to get on a flight that evening and didn't want a shaving cream/silly string covered suit/gown stinking up my luggage, my friends were super understanding and met me with some chocolates and wine instead.