Wǒ shàng zhōngxué de shíhou yě dāng guo fēizhǔliú.
I went through a "non-mainstream" phase myself in middle school.
Tips
usage
非主流 is the opposite of 主流 'mainstream': 非主流音乐 is 'alternative music'. Used as a noun, it means the youth style of the late 2000s or someone who followed it.
culture
Around 2008 the 非主流 look filled QQ Zone: side-swept bangs covering one eye, big eyes, heavy filters and captions in jumbled characters. Its louder cousin was 杀马特, from English 'smart'. Today calling an old photo 非主流 is gentle teasing.