I was a primary teacher and in my time we have had 6 diabetics in school. We always had the testing kit and snacks in class (cloakroom just outside if privacy required) and when testing an adult would oversee (even if it was just me overseeing while also monitoring the class). If low they would eat in the classroom at their desk.
At lunchtime, they would start the year (Y3) generally on 3 injections a day so only needed to test at lunchtime. This would be overseen by a first-aider. If they were particularly high then parents would be phoned to come and give extra injection. By the end of the year they were generally on 4 injections (It's just the way it worked) so would test and then inject themselves at lunchtime with the correct dose being dialled up by the first-aider.
We had a sharps bin in each classroom which had a diabetic. One thing I would say is give the teacher a very clear table showing what to do at different readings - all our diabetics had different levels and 'rules'. Also, a bit of 'give and take' is handy - most of our diabetics had a simple 'if below 4 then glucose and snack then retest 10 mins later, if above 18/20/22 phone mum or dad, otherwise just eat lunch as normal.'
However, we had one who had a very much more detailed list - which included if he was a little high but not hugely high he had to run round for 10 minutes then retest, if still high then run round for another 10 minutes before eating lunch. We followed this,but it made life very difficult because he had to be supervised running around (not least cos he hated running!) which took a dinner lady/TA away from the class they were supposed to be supervising, and then, when the rest of the class were ready to go out, he would come in for his lunch. This isolated him quite a lot.
Now, I don't know if that's a normal thing to ask the school to do or not, but the other children had never had that as part of their routine. I guess I'm saying make it as easy as possible for the school to deal with and they will deal with it as best they can.
Sorry, this turned into an epic!