Hi OP,
If you want to be invisible, that's great - go for it. I think it can be a great choice at the school gate, and I go for that option often to avoid getting dragged into stuff.
However, if you don't want to, it's a pretty easy fix. There is obvious stuff about slightly adjusting your dress to be maybe bolder colours, slightly smarter, more formal, less rumpled,., but most of it tends to be posture.
Stand up straight, shoulders back, with equal balance on both feet. I find it's very cantering to imagine your hip area is grounded, with a strong connection right into the earth. Breathe slower and deeper from your diaphragm. This will make you look more confident and also give your voice more authoratitive depth.
It can also help to imagine there is a string attached to the top of your head going up to pass over a hook in the ceiling and back down to you... just imagine gently tugging on the string so you are using your full height and looking outwards not down.
Then there is being open to the world - a mental attitude but reflected in making eye contact for example, in a cafe with whoever should be serving you.
Then what do you actually say? Make sure you aren't using lots of diminishers (we women are often brought up to)....stuff like "could I just..." "not sure if,... but maybe..." "I'm just a little unsure whether..." or phrasing things as questions when actually they should be statements. Keep sentences relatively short and lean on key words, with eye contact.
I guess I'm going into this because it sounds a bit like you feel 'being seen/noticeable' is an innate quality you have or you don't have,.. whereas I would suggest it comes down to actual actions. The visual, verbal and vocal cues you send about whether you deserve and are comfortable with attention. You suggest that deep down, you're not sure you are. And probably right now, you are communicating that in how you dress, stand, move, speak etc...
As I said totally totally fine to do that if you want. And the school gate really doesn't matter much. But if it bothers you, you can do concrete things to change it... it can actually help change how you feel inside too.