Thirty students isn't a particularly small sample but what might be the problem is that your boss has been reviewing data for 10 teachers for 5 years, that's 50 times she has "tested" a teacher's data. The chance that an individual teacher's data looks unlike a bell curve might be small but the chance of finding one odd set of data after looking at 50 teachers' data could be very high.

If you wanted to formally test that your students' grades don't follow a bell curve you can use

this online calculator for a statistical test.

The problem with this is that your students' grades never really follow a

*true* bell curve, a true bell curve has possible values up to infinity but your students wont get over 100%. With only 30 students I doubt that none getting grades above 100% would be a problem; what could be a bigger issue is if you have grades A,B,C,...F instead of the percentage for each student.

To avoid this issue it's a good idea to compare the result of that online calculator with results from other classes/previous years which did have an acceptable bell shaped curve.

I hope this helps