Euphonium / Baritone

If available I highly recommend you hire for the first year.
Euphoniums and Baritones are fairly expensive, especially compared to Clarinet, Flute and Trumpet.

Small students will begin on a Baritone, but will wish to move up to a Euphonium as the grow. The Euphonium offers a larger and fuller sound. 

As students progress, they will want to end up on a 4 valve Euphonium.
Good high school band programs have 4 valve Euphoniums their students can hire.
Some Euphonium players choose to move onto Tuba when they are big enough, enjoying that gorgeous deep and warm tone that makes a band sound so incredible.

Yamaha YEP 201

(Recommended)

Current Street Price with an Educational Discount: $ 2,449

Yamaha YBH 301 - Baritone Horn 

Current Street Price: $ 2,499
Ideal for smaller students

Intermediate Model

Yamaha YEP 321

(Highly Recommended)

Current Street Price: $ 3,299
Requires larger hands, play test this instrument for size before buying.
The tone, projection and volume of this instrument is exquisite. 
This is a very popular instrument for Professionals who double (meaning Eupho is their second instrument) so can't justify spending the $10k required to buy a top level professional Euphonium.
I personally played a YEP321s and loved it, until I got a phenomenal deal on a YEP 621 (the entry level professional Yamaha Eupho)