You can sign up for a teacher account by emailing email@example.com -- be sure to email them using your work email -- they'll ask some questions about what organization you work for and how many classes containing how many students you want on your teacher account. Then they'll send you an access key. After signing up and creating your learntomod account and entering the key, email them back and then they'll upgrade your account to a teacher account.
The minecraft launcher just needs to be installed on your grandson's computer. It needs to be the right kind of minecraft launcher, specifically this kind: https://learntomod.freshdesk.com/support/solutions/articles/5000592008-where-can-i-download-minecraft- If you have a paid minecraft account, you may be able to download it multiple times from minecraft.net without paying more. But remember,you can't be logged into minecraft at the same time as your grandson, using the same minecraft username, otherwise you'll see an error. If you're seeing that kind of error, that means you need to buy a seperate minecraft account for him.
The learntomod website automatically detects your minecraft username the first time you press the start server button on the website. But you can change the minecraft username saved on the website to something different. Just click the account gear icon in the middle of the learntomod homepage, scroll down until you see MC name and click the blue change button -- then enter the minecraft username you see on the minecraft launcher installed on the computer you're using. The minecraft username usually isn't an email. It's located in the corner of the minecraft launcher when you're logged in.
As far as I know, your grandson can't be logged in to use two servers at the same time. You could wait until the class is over or you could ask the teacher to remove him from the class, then he'll be prompted to enter his paid access key when logging in to the learntomod website. The paid access key will give him access to our minecraft server, assuming he bought a subscription on learntomod.com. If you bought a subscription, it will autorenew once a year -- if you want to turn off the autobilling, email firstname.lastname@example.org with your receipt at any time.
Another possibility -- if he wants a fresh start -- is to let your grandson sign up for a learntomod account on https://mod.learntomod.com with a different email address as his login. That is, the login email could be different than his create and learn's learntomod account. His data and mods and such are tied to his login email. If he's already used his paid access key, though, let me know. He can only use the key one time.