We use 4matrix. Got around the problem of it not being web based.. 4matrix works OK over our school VPN.
Online speed is a little slower than in school but acceptable. Staff have the 4matrix client installed on their school laptops (and obviously the VPN client too) and can therefore access 4matrix from anywhere they are online. But they can only do it on their school laptop, and not on their personal devices. This restriction is not a problem for us, but might be for other schools.