I would agree with what Mike has posted. Particularly for a school program, marching baritones are much more preferable compared to most marching euphoniums, especially in regards to ergonomics and ease of use. The two summers I marched DCI, the group I was with used Jupiter Quantums, and then the King Ultimate series. Both marching euphoniums played well, but are quite heavy and large. I wouldn't recommend them for most school programs, for the same reasons Mike mentioned above.
BD and Phantom Regiment are the two corps that have/still do use all euphoniums in their hornlines, and a few like SCV and I think Boston Crusaders and Cavaliers use all baritones (Yamaha 301Ms). But it isn't unusual for groups to use a mix of them.
The Yamaha 301M can provide a characteristic euphonium sound, and is compact and lightweight enough to be easy to hold and manage for students. Part of what helps with this is the design of the horn, where the valve block is positioned closer to the back, so the weight center of the horn is closer to your body, making it easier to hold with great posture for longer periods of time.
Another option that could be good is the King K30 (formerly System Blue's SB30 before they split with Conn-Selmer, changed the design and shifted manufacturing overseas). This model is a "hybrid" marching euphonium, so it's a little more compact and easier to hold than other marching euphoniums, but is still larger than the Yamaha 301.
Willson 2900 TA-1 Euphonium - Denis Wick 4AM
Yamaha YSL-643 Trombone - Bob Reeves BrassArk 5G "Gladstone"
Yamaha YSL-8440 Trombone - Denis Wick 5BS
VMI 3301S BBb Tuba - Schilke Helleberg
Past:
York Preference 3067 Euphonium - Denis Wick 4AL
Benge 165F Trombone - Benge Marcellus
Wessex BR140 Baritone - Denis Wick 6BS
F.E. Olds Special Trombone (ca. 1941)