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 IMPEDANCEXLR 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).
give details on the show. For example, it would work fine for a skit with an audiencee of maybe 20 or 30 but it wouldn't do at all for a full length play with an audience of 100 in a huge gym or auditorium.
When they do an instrument amp like that, with a built-in speaker, they usually design the amp for the speaker they're planning on using. Hooking it to external speakers may be a problem - especially if you're planning two or more external speakers in parallel. The lower impedance will make the amp kick out more power... for a while. But while it's kicking more power to the speakers, it's also turning more power into heat inside its overworked transistors. It'll work, sort-of, for a while. But "a while" might not be long enough for the whole show. And for any venue larger than my living room, 100 watts RMS isn't much to work with. I'd see if I couldn't find a rental outfit to cut me a deal on the use of a real amp in exchange for featured mention in the program notes or something.
A PA is a PA, Backline is Backline. However, PA is NOT Backline and neither should Backline be PA. Good rule of thumb to follow. Even in dire straights it's better to have your musicians scream their vocal chords raw than put a house mix through a keyboard amp.
100 watts will be just barely enough for live music with a live band, I think you are doing more than just cutting it close!!! Yes, you could easily ruin your amp, yes, if you have a large room to fill you will NOT fill it! 100 watts would be ok from a real PA amp, one designed to run to passive speakers, and even then I would tell you that you need 100 per side, not 100 total. Get something bigger dude, really, try to!!