@henman Yeah, You're definitely right about the middle school part.
Whenever Yuzu uses the -masu form of a verb or a copula around someone she is not comfortable with, it's written in the katakana, and she says kudasai as くだサイ, and converts a number of other simple exclamations or common phrases to Katakana. My guess is that Atou Rie is only making the non-meaningful parts of the sentence into katakana for the sake of readability.
I think in this case, since the た is a conjugation suffix on ます, it is left as hiragana, which would explain the franken-word of ありマシた. This is corroborated by the fact that slang terms written in the katakana still write inflected parts as hiragana (like Namekawa's ナメる).