this doesn't look very yandere, someone tell me what i'm missing

Well you can't see it here that's for sure.

However, they don't get much more yandere than Yuno so I can understand why it was tagged as such...

We don't tag personality traits that aren't visible in the picture.

Do explain why this is tagged with "school uniform".

Yuno's the type of Yandere that goes after anything that threatens her prospects with the guy she likes, not the guy himself; with him, she'd be very loving.