Hypos can scare you, but mostly, they are not all that dangerous. it's no wonder your BG went up so fast after a Mars bar!( was it a mini one?) I know they're delicious, but Glucose tablets like Dextrosol, would be better for you as you can make sure you don't have too much.
Ask Mum to ask your doctor if tyou can please have some Hypostop, for just in case you have a sudden and severe hypo.
Remember how you felt when the hypo started to come on. That's your warning sign. that's when you need the glucose tablet, to prevent the hypo getting worse. If you take the glucose, test after an hour and you should be normal.
Another hypo preventer is Lucozade. It works pretty quickly. You can have some in your school bag without the other kids wanting to know what it is.