Assuming your mask fits well to begin with and you don't have leak problems in general-
Having a reg in your mouth tends to change the shape of your face a bit. (It's generally suggested that you have a reg or snorkel in your mouth when trying on masks because of this, you don't want it leaking when you DO have a reg in your mouth!) So maybe your mask fits better with the reg in your mouth than without. You could keep this in mind in case you are doing something much different with your mouth.
Or water might be entering because exhaling from your mouth is causing a bit of turbulance that tends to dislodge the mask. Try doing it more gently.
Realize that you don't remove the reg from your mouth all that often after class. Also realize that water will get in the best fitting mask from time to time - it gets bumped, a hair sneaks under the skirt, you do a drill and switch regs or something, or whatever. That's why mask clearing is such an important skill. Believe it or not, while water in your mask may really bug you now and mask clearing may not seem trivial yet - soon enough you'll find a little water doesn't bother you much, and you will automatically clear it while hardly even thinking about it.
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