"He's my son and I am angry that the school never took into account that I came to them." - They did take it into account. They literally responded. You told them he smoked cannabis, and they responded to this. Do you think he should have got a 'get out of jail free' card because you told on him? A word for the future, if he ever commits a petty crime, don't report him to the police, hoping that they'll go easy on him because his mum told on him.
"They didn't catch him smoking cannabis." - They didn't need to. You literally told them yourself!
"He was told he was being suspended DURING his exams." - Because YOU TOLD THEM at this time.
"That if there was any hint from him OR anyone else that he has anything further to do with drugs he would be expelled." - Yes, as is the case for everyone. All the children you reported will be in the same boat. Why would your son get special treatment? The first time is the warning. The second time is the proper punishment. You reported him the first time so he lost his warning and has gone straight to the next step, where is he is caught he will be punished.
"As I informed on the boy who supplied the drugs who also was suspended (because it boiled down to 'there was no proof') I imagine the other boys might not be too happy about being informed upon and possibly do something about it." - The boy who shared his drugs with his mates, one of whom was your willing son. Stop using language to imply that your son's friend is some kind of dangerous drug dealer prying on young children? He's literally the same age as your son and shared a bit of weed with his mates. And yeah I doubt they are too happy with him since he went home and told his mum and now they're all suspended? Surely you thought about this before your reported it to the school - the effect on your son?
"He is 16 and can make his own decisions." - Well... he can't, can he? You literally reported him to his school for sharing a joint once.
"A suspension is held on file. If he applies to another school they will be informed which might hamper his chances of getting in." - Again, didn't you think about any of this before reporting him?
"I don't believe he should have been suspended because he came forward with the info." - Why? Why should he be rewarded because his mum told on him?? I don't understand this at all.
"His nan died. Have a bit of compassion." - The other boys might have had their own issues too? But you reported them, are happy about their suspension and punishment, and seen to have no compassion for them? This is all a bit hypocritical. You seem to be saying that just because he's YOUR son, he deserves special treatment. He is being punished for breaking a school rule. It is as simple as that.
"But my concern is with the school and their drugs policy. That their policy does not mention what would happen to the child if a parent comes forward." - Why does it matter who specifically reports a child for breaking a rule? If anyone reports someone for breaking a rule, and it turns out to be true that they broke the rule, they will be punished. Obviously. Why would they need a special clause in their drugs policy detailing what happens when certain people report them? It's the same for everyone. If a classmate reported him it would probably be treated in the same way. It's not about the report, it's about the fact that he did the thing reported.
"Which is why I asked you if you would still report to the school if you knew your child would be suspended." - No, of course not. I wouldn't report my child to the school for smoking weed one time and being honest with me about it even if I DIDN'T think he'd be suspended. I'd let him know the risks of drug use but also thank him for being so honest with me which is a really big thing to do.