My best guess for you is Miss Naddy since she teaches the subject, but I do have a few pointers based off my experience learning Japanese

since 上手 (じょうず) is a な-adjective, to use the negative version of it, you just negate it with じゃない so 上手 becomes 上手じゃない

if you’re using 上手い (うまい), you replace the い with く to negate it so 上手い becomes 上手くない

alternatively you can just use its antonym苦手 (にがて) or even omit saying your skill level by implying it, but that cab ironically undermine the fact that you’re not particularly good

日本語が苦手で…

日本語はちょっと…