Have briefly been on a school trip to Prague about 5-6 years ago, but we didn't see much at all.
We were able to get a good deal which included BA flights (£160 for 3 nights accommodation and flights) from LHR, which I thought was pretty good for mid-May. Next best thing would have been from STN with CSA, but terrible flight times (late evening departure, early morning return).
Fingers crossed all should go well and a fun time shall be had by all four of us!