Playing random kids at your school isn't really representative of competitive Smash, these are just people who like the game - not professional players. Beating them, while cool; just isn't a true mark of skill.

If you really want to improve, then you have a few things you need to commit to. Firstly, is settling on a character that you want to main. The reason for this, is that it's much better to focus on the meta of a single character - in order to optimize improvement and practice regimes. Then, submit a recording of your gameplay to r/CrazyHand (competitive smash board). There are lots of good players there who are more than happy to advise you on how to improve. You may need to do this a few times over a period of a few weeks/months, to see if you're actually absorbing the advice. Finally, finding a character specific discord page will put you with a character based hive-mind. I've yet to come across one that wasn't friendly and understanding, providing that you act cordially yourself.

Good luck, man. The grind is a long one, but it's entirely worthwhile once you start seeing the results.