As many comments have stated, you have a knack for melodic writing, and that is fantastic. As you have written for marching band instrumentation, I will comment from the standpoint that this is to be performed during a halftime show, however most of my suggestions should help you regardless of what type of ensemble you write for. All beginning composers all have difficulty with balance and blending. For example, the clarinet solo at m. 32 is lovely, however you have to remember that the clarinets will not be heard on the field especially with low brass accompaniment. You can either double the solo on other instrument(s), or score the melody for instruments that will project over the texture and across the field (e.g, brass, high woodwinds, pit percussion (xylophone, glockenspiel)). During that same clarinet solo, the marimba is doing some cool things. However, marimbas are rather quiet unless they're mic'd (which most high school marching bands don't bother with), so maybe write that marimba line out to something with more punch. Finally, keep in mind like adding triple forte for woodwinds and pianissimo to brass won't give you the desired effect you want in real life. Balancing the sections to bring out the line is more important and comes with time as you grow as a composer and orchestrator.
Notation is another big thing that beginning composers struggle with. Being able to convey your music in the most effective way to players, is the name of the game. Composers do this by following a set of notational conventions that you learn over time and teaching yourself. I highly recommend the holy grail on all things notation, Behind Bars by Elaine Gould. Many college professors I met also use this book religiously, and the language used is friendly to beginners, too; if you are serious of pursuing composition in college, I'm sure that you will invest in such a valuable resource as this book, as well as other orchestration books. Anyways (I'm gonna ramble here), small things like using appropriate note value durations, adding that fermata to all instruments in m. 38, avoiding whole note slurs in m.18, etc. These little things are learned over time and from reading notational resources, like I mentioned.
[I won't comment on the harmonic choices as @Tónskáld has already mentioned many.]
I am currently a senior in high school looking to pursue music composition in college as well! I began writing things just like this, and it is amazing how much you can grow. As long as you keep educating yourself, doing score studies, etc., you will improve dramatically. Just this year alone, my writing has increased 10-fold, and I won a composition competition for a youth orchestra, and my piece will be performed live at Orchestra Hall this May! And that is from being involved with the composing community (American Composers Forum, Reddit, YoungComposers, etc.) In conclusion, my advice on this front is to be as involved as possible, get your pieces played by live musicians, and to hone your craft.