You didn't get the grade that you really got, so
1. It's morally wrong
2. On test day, when you have no idea what's going on, what are you going to do?
^ This. She (or he) obviously trusts you guys enough to grade your own papers. Giving yourself A's all the time doesn't benefit you in the long run, and it will be evident that you didn't deserve the grades when it comes time for the actual tests.
Honesty is the best policy.
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My only teacher to do that lately is my Art Teacher, but I always critic my art harshly. She usually ends up giving me a higher grade and writes, " Stop criticizing yourself so hard."