flight postponed

i now have 15 hours left before my ppl checkride. tonight i went to do some hood time. the 150 that i have flown since i began was out for its 100 hr check. so i was going to fly the other 150 that the school has. well did the preflight and fire it up. when i did the mag check the left mag caused a 600 rpm drop and it was running real rough. taxi back to parking and tied it up. noway was i going to fly that with a sick engine.