If you know 100% what you want, (ie. know colours, shapes, magazine pics.. favourite ma you've worked with before) it's a great idea. They are absolutely professional and for the most part should be able to execute what you want as long as you talk with them. If you don't know 100% try to talk to an MA for ideas or do a run through beforehand, but remember you will most likely have to pay for the second one.
Keep in mind on the day of your prom/grad.. a LOT of other girls are going to be there as well. The MA's will be busy which can translate either into super exuberance (which is fun and exciting) or annoyance and irritation (which does happen.. and isn't really that great). I for one wasn't really up for the stress to hell day so I chose not to go with MAC for grad. I went with MUFE who were absolutely amazing. They're a lot lower-key than MAC since not EEEEveryone knows them. It was great to go since I had the MA's full attention for over an hour and a half (in fact I had 3 MA's helping me when they got quiet). Since it was so quiet I was able to run a couple of ideas by them and discuss everything I had problems with.. from colours, to textures to actual products. But either way, MAC or someone else... it's a great experience.