Stuttering is usually influenced more by scenarios or environments that become more anxiety inducing ie. being around people, presentations, when you're more emotional or angry etc. so it's completely normal to be comfortable and fluent if it's just by yourself. Unfortunately it's neurological so there's no quick fix to get rid of it and there are going to be times that make it worse than normal and most people have come to terms with this but you are young and it's possible to retrain how you speak if you are looking to diminish it a little bit - look into smooth speech tactics (slow starts, elongated talking, slowing down your speech speed), and also noting down what scenarios you stutter more in and exposing yourself to it more so you can practice and lessen the anxiety surrounding those scenarios. Set a goal where you use your speech tactics on the first conversation you have of the day and practice self care and love so you don't take any embarrassment to your self worth and confidence. At some point you may have to accept this is part of your identity and isn't completely fixable but high school is short, i promise that people are more understanding and compassionate in university and adult life. You'll get there!!