All that I can say is that I feel for you!
Maybe people do things differently in Spain, but having over 100 PCs in an educational environment with no central security system must be the biggest pain in the rear.
It might not be too bad if you have computers setup with the same Username and password on each one and use something like steadystate or deepfreeze to keep them clean, but what about your user's files? What happens when you have 10 students assigned to each PC and one of them goes down? How do you keep their files backed up (a backup for each computer? TimeMachine for Macs?) or do you let them use flash drives (what if they lose them)?
I just can't even fathom how much extra work you'd create for yourself if you didn't have a domain or some set of central management to keep user ID's, passwords, and files in and to keep them safe.
In a domain, any student can login to any computer and have access to their files, which are backed up three times a day (two shadow copies and a full backup at night). If a computer goes down, they just login to another one and continue working. We can audit the computers to find out what student was logged in when they do something wrong, and we have the power to leverage software like ADNM to push out Avast and monitor each computer, not to mention the power of Group policy to lock down the computers for certain users and push out other software like Microsoft Office.
Sorry, I just can't see anyone trying to use a workgroup with that many computers. Your customers must be in the minority, and most companies / organizations with 15 users or more must have the foundation (of a domain or other central system)in place already and can use the SBC or ADNM solutions.
I'm not saying you're wrong, but I don't think it's Avast's fault that their "Managed Product" doesn't work well in your customer's non-managed organization.