It's a big city, but I would not worry too much about violence in the tourist areas during daytime. Just put your wallet on your body or in the side pocket of your jeans. Don't pull it in the street. Mind your bank cards. They're more interesting than cash nowadays. Be careful when you cross the street. Also, people might ask for money or offer you drugs or a (stolen) bike. It is best not to engage these people. Of course, if you go near the red light district there will be prostitution out in the open, so be mindful of that if you're the sensitive type.
Lastly, don't get wasted and go swimming in a canal. Only the locals do that!
Really, if you're sensible, probably nothing will happen and you'll have a good time.
Amsterdam is very safe city. But if you do stupid things no place is safe on earth.
Pickpocketing is a thing in every large city in Europe. That's a thing to keep in mind specially with the Eastern European pick pocket gangs.
https://www.youtube.com/watch?v=6BeoYa1w...
Where are you staying? If you're all legally underage, you won't be able to get a hotel room because you won't be able to legally sign a contract.
As long as you stay clear of the dangerous parts of the city (back exit of the train station etc) and enjoy your time responsibly (i.e. don't get so drunk and stoned on drugs that criminals can take advantage of you), then it is one of the safest places in the world.
No hotel or hostel will rent a room to a minor.
