I would suggest contacting your union as a matter of urgency as they have been extremely useful for me over the past few years. HR in my school have been terrible. Sadly SLT and HR in schools seem to be ignorant of the condition, the needs of diabetics and the rights that we have in the workplace. It took me 3 months in the autumn term to have a decent risk assessment put in place - the new timetable has meant a 20 minute lunch break was imposed, my levels inevitable spiked massively after lunch and I was having bad hypos around 11am each day. They did not seem to understand why I couldn't toe the line like everyone else. I now have someone from another department who babysits my group at the end of period 3 each day, so that I have 35 minutes in which to check levels, inject, eat and prepare for period 4.
If you are in a school with a union rep, speak to them. If you are in a school without a rep (like me) contact the union directly. There are many things that schools will put in place that go against statutory pay and conditions, but if no one stands up and says something it becomes the norm.