I would go with Smoothwall (or a similar product such as components from the Fortinet suite). Although these products are expensive for schools, they do have the range of resources needed to tackle all the modern filtering/firewall needs. Tackling the dynamic threats we all face is well covered by supporting unified threat management ( see: https://en.wikipedia.org/wiki/Unified_threat_management ). To effectively manage our internet service does require the tools for the trade, and I feel Smoothwall is an excellent product and has excellent support. With an appliance on-site and cloud-based components this does ensure that I can manage my own issues very promptly - a block in place in 30 seconds.
I used RM SafteyNet until a couple of years ago. Getting changes made could be a bit too slow for my liking, as the service was entirely cloud-based and necessitated email or telephone calls to get a concern looked at. Some delays would occur while a panel of experts would discuss whether a block was acceptable, based on consensus. The system was simple - static black lists, white lists of URLs and IPs. Due to the simplicity the service was very cheap. I haven't checked it out since 2014, but it desperately needed a means for end users to get an immediate block in place and some functionality for dealing with man in the middle attacks, deep packet inspection and other dynamic tools.
I suppose it comes down to you get what you pay for. The modern tools like Smoothwall do provide the comprehensive suite of features for monitoring and managing your filtering/firewall. In Smoothwall's case their support in excellent (I have just spent time at the BETT Show in London talking to quite a few colleagues on this topic, and they all agreed Smoothwall support is first class).