。
It's for vocational school.
Literal
Specialty school [attributive-の] sake [copula-だ][assertive-よ].
Very casual, youth-style speech. ガッコ is slang for 学校 (a truncation with 促音 stress), and タメ is slang for ため ('for the sake of'); writing them in katakana reinforces the informal, colloquial flavor — this is how anime/manga or youth social media renders casual speech. 専門学校 is a 'vocational/specialty school,' the post-high-school alternative to university. The full formal form would be 専門学校のためだよ.