The schoolgirl outfit wasn't a DLC option. Kat having to pose as a student for a bit to gather information is an aspect of the main storyline. The small arc that takes place around that school is a pretty good character moment for her in general.
And it's a very reasonable
design as far as schoolgirl costumes could go, imo. I'll let you be the judge, but personally I think it looks like a perfectly normal outfit. And to head things off at the pass, no, you can't peek her underwear in this either.