How can I allow students to use a classroom ipad or computer to do DuoLingo? Do they have to have their own account for that?
Your students will need to be logged into Duolingo if it's going to be of any use to them. On a computer it's really easy; they log into the machine, and then they log into their Duolingo account. Depending on how your school's computer systems are set up, it might even be that the web browser remembers that they're logged into Duolingo, as it does on a non-networked computer.
As is often the case, if you want to try to use tablets in your classroom, it's going to be a lot more difficult. Tablets are designed as single-user machines, so you'd have to be very careful to make sure that each student shows you that they've signed out of the app, otherwise it's going to get very confusing and frustrating very quickly.