So I am a sophomore in high school, and I play a Wessex "Dolce" EP100SG. I want to improve, and I've made it very clear to my BD that I want to, and she is trying to help, but is just giving me information I've already found (she wasn't a brass person) and I've been considering buying a method book. I've been recommended Arban's, Clarke's, Koppasch, and others. Will this be helpful in producing what I want to do with my horn, or should I take classes (or attempt to, there's most likely no one who teaches Euph in the area) and proceed from there.

Here's a list of what I've been told:
Method Books
Solos
Competitions
Learn additional clefs (aside from treble and bass)
Listen to soloists
Look for a community band
Scales and chromatic scales
Transposition
Sight-Reading
Do long tones and pedals

I have a stable range from G1-C5, and I can extend that to Db1 and G5, but it's extremely unstable. I play bass clef, but say most of my things in treble (I've gotten used to it, I am the only bass clef reader in my section) and I read both. My major scales are fine, I play most of them daily, and my minor scales are not the best, I only know A and F minor.