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Our gym clothes used to be normal shorts, but starting today they suddenly switched to bike shorts, you know.
Literal
Our-school [genitive-の] gym-clothes [casual-topic-って] normal short-pants was [despite-のに], today [from-から] suddenly bike-shorts [into-に] became [ended-up-ちゃった] [explanatory-の][confirmatory-ね].
A teenage complaint about school-uniform changes. うち here means 'my school' (similar to its use for 'our household' or 'our group'). ~って is the casual spoken quotative/topicalizer, replacing は or という. 短パン is a clipping of 短パンツ ('short pants'). スパッツ is a loanword from English 'spats,' here meaning 'bike shorts' or 'tight compression shorts' — a common school PE uniform component in Japan. ちゃった is the casual contraction of ~てしまった (completed action, often regretful). Together the sentence has multiple markers of a teenage girl's casual speech: って, なっちゃった, のね.