When you tune as an ensemble, do you have to pull the tuning slide way out? If not (compared to the others) then you're probably not lipping sharp. Interesting that things were ok until you got more practice and experience playing at greater volume in marching band. You really need to consult a specialist brass teacher but I suspect the rest of the section is playing in a dull, timid way. Trumpet is supposed to be bright. I don't think everybody is really talking about it. If they are, they must have very boring lives.