Some clothes are more forgiving of fluctuations / changes than others.

Tights are great because they expand. Spandex-y stuff. Typically come in small or medium or large so you probably won't need to upgrade anytime soon

Flowing stuff without waistbands... dresses / long t-shirts to wear over the top... Sweaters. T-shirts etc.

Anything... Stiff and tailored is much more sensitive to sizing fluctuations.

I'm a fan of gym gear because I find it comfortable. Am a student... So can get away with hoodies for cooler months. Jeans are a bit tricky... I've never found much of a way around needing a few different pairs just in order to deal with monthly fluctuations, never mind anything else.

I avoid skinny jeans and go for a looser cut. But I get that teenagers have a bit more social pressure on them to wear stuff that helps them feel like they fit in. Do you have friends who can shop with you / maybe exchange some clothes etc?