I tend to not talk about my personal life on the Internet but this one seems like an easy environment to talk about it so here we go.
If there's a bad habit I have, it's probably my clear lack of organization.
I have a very bad tendency to spread my stuff around a given space and a very bad tendency to often get distracted. Naturally, this hurts my productivity in school [and probably college and work once I get to that stage in life] and also makes it easy for me to forget stuff. To be fair this is because I have ADHD but still it's a habit that's noticeably stuck with me.
Despite this, I still tend to do well in school as it doesn't really affect the quality of my work particularly severely, but it's been a habit that's stuck with me for a long time and it's something I've wanted to overcome. The support of my parents has helped in this regard, however.