Intonation is tricky. There are two basic concepts you can use to help. One is recognizing "overtones" or the "buzzing" as one person called it between two pitches. The other is aural memory.
To work on recognizing overtones and adjusting the pitch, I recommend a
Tuning CD. You have to be able to recognize overtone "beating" between out of tune pitches and then be able to adjust your embouchure to "fix" intervals. The Tuning CD gives you pure tones that you can easily tune your trumpet to. You can use a piano as well, but it's difficult to hold the key down and sustain the pitch while you try to tune a note with your trumpet.
Secondly, you need to work on aural memory. This is being able to "remember" where you moved your embouchure to get a note in tune so you can play it right the first time without having to constantly "scoop" notes while you play. Student ensembles have a very difficult time with aural memory and typically play out of tune a lot. Having sectionals and focusing on out of tune intervals in the music is the best approach. The trick is to "remember" what you did in sectional and then apply to the rehearsal.
Good Luck!