As with most instruments, trumpets have certain notes that will sound out of tune if no adjustment is made by the player. Proper intonation is dependent on great air support. Have students practice high notes using the strobe tuner. This helps them realize how to properly play those notes and also trains their ears on how to analyze their pitch. Eventually they will learn to make the necessary adjustments to play the full range of notes in tune.
Fatigue is also a consideration.
As students become fatigued they often use too much mouthpiece pressure. Using only enough pressure to seal the lips to the mouthpiece will allow for increased endurance and properly tuned notes. Tuba Low-pitched instruments such as tuba take longer to warm up. Often times, it can take as long as 15 minutes for a tuba to warm up completely. If the tuba is tuned before warming up, it will go sharp.