To indicate how serious this is, being caught with just 2 grams or more of dexamphetamine is enough to be charged with supply under the WA Misuse of Drugs Act. So yes, selling this stuff is very serious.
You haven't said anything about this, so I will assume the worst at this point, in that you sold the drugs to fellow underage students and that you did so on school property.
Although you don't say if you had any of the drug on you at the time, you did confess to selling it, so that in itself is an admission of supply. I don't know for sure, but I would think that the Police could charge you with supply based on the admission you've already made to the school, regardless of the amount you sold. (Someone will correct me if I'm wrong about that.) So yes, you should speak to a lawyer and you should do so before speaking to the Police.
The other serious issue you face is what action the school will take. Unless there's a law that prevents it, I would expect that you're looking at expulsion. Drugs in schools is an extremely serious issue and if the school doesn't act accordingly and word gets out, then there will be backlash - and the school will be well aware of that. (Think of a few hundred parents bashing the principle's door down threatening to pull their kids out of the school. Extreme example? Maybe, maybe not.)
On a personal note, I hope your mum gave you a severe clip around the ear and that you've learned your lesson. You're far too young to get involved in this sort of thing. It may seem harmless at the time, but it's illegal for a very good reason. Dealers always start off small, but there's only one road from there - and it's all downhill. I hope you can see that now and that you're smart enough not to throw your life away. At the end of the day, it simply isn't worth it.
Best of luck.