2 needed to calculate an average
Both Full Metal Panic! and Wild School feature a mercenary trying to blend into a school environment. Characters genders are reversed in those two series. FMP! has a male mercenary and Wild School has a female one. The stories themselves are not really alike, but if you think the thing I stated in the first sentence is interesting, then you should give both of those a go.
If you're looking for more of those teenage female soldiers placed in a high school setting, but with some comedic mixed in, then Mission! School might suit your taste. Because it has both those warrior girls and comedy. The second one is even provided large quantities.
If you think that a teenage female soldier thrown into the unnatural environment of normal high school would make a good plot of a series, then School Shock and Wild School could spark your interest, because they're both exactly about that.