First of all, ear training the alpha and the omega to musicians. Because, a musician without a good ear, is like a driver with no eyes. Sure, you can drive, but where are you driving at? So it is really important to train our ears. Usually you can find one of those software for less that $100, especially if you target software for your personal use. On the other hand, if you are looking for software for you class, or for your school (as a teacher, of course) you can easily find some products that support that kind of need. For example, you can find software that can register to an entire school, for 20 computers, or more. So you don't have to worry about that.
Certainly, if you want your students to have trained ears, the school computers must have headphones, because if you have 20 computers with an ear training program in the same room, and have speakers on them, it's just going to be hell! Imagine 20 students who are doing different exercises at different times, trying to hear their own example over 19 other programs that play music. It would never work. So, headphones are required. Don't try this experiment without having them.
You can also set up a single computer, if you budget is limited, and the students can create different accounts in the program, with passwords and everything. That way it will come out much cheaper, but much less practical, because only one student at a time will be able to study and train his ears. In addition, if you want the full experience of the program, you should also consider providing the computers with microphones, because the programs usually pay a lot of attention in singing correctly, and they have ways of identifying if the student is doing so. Of course, it's no big deal if you can't buy a dozen of microphones, and then you won't even have the trouble of having to listen twenty students sing awfully in the same room.
My advice is to try the program yourself before buying the full program for the school. Maybe you can even download a trial version, and see how it plays out. Because, imagine what it would feel like if you bought a software for the school, you paid $700, and then you realized that it was not worth even $100!
So, be sure not to do hasty moves, research the market first, before you buy and music software, make sure that it's exactly what you want, and that it's worth it, and then buy it for yourself, your business, your school, or whatever you want to buy it for. My best wishes in you search for the best ear training software online. I hope you will find what you are looking for. And always remember, no hasty moves!