I would strongly recommend not making such a move - it would be better to ask your principal or some other organization with a large budget for some money to buy proper, dedicated equipment. You might even consider writing some grant applications if your department is financially strapped.
Technically, though, it suppose it would work - but it would be a waste of money that could go to a quality Crown/Crest/QSC amp and a good pair of speakers. Make sure it has a LOW IMPEDANCE
XLR or LOW IMPEDANCE BALANCED
TRS (1/4") input - an UNBALANCED, HIGH IMPEDANCE (i.e., guitar) input is not good and could possibly damage your system (to what extent, I am not sure - but I wouldn't want to risk it).