I don't think there is a lot you can do.
One thing I've read is that worried parents calling their children on their cells actually give away their positions. So it would be probably be a good idea to shut it off.
Always make sure you are in a good physical health, to maximize your escape chances.
Packing a pistol is maybe a viable option in some US states, but almost illegally in Western Europe
I saw an overhead picture of the tiny island. There is one building and 50% trees cover. He could roam the island with little impunity for quite a while. He choose his killing field well for his purpose. And because of the short distance to target and no real shelter he could get away with lower accuracy by using high frangible bullets that cause a lot of internal injuries and bleeding. He wanted those kids to suffer as much as possible.
No, in this situation, there wasn't much they could do. A few of them played dead or covered themselves with another body but other than that it was basically wait for help to arrive. Some tried to swim away from the island and he shot them in the water. Some kids barricaded themselves in the one building and put up mattresses against the walls. The killer also wore some sort of police uniform which confused the kids as they thought he was there to help them.
The killer also didn't resist with a fire fight when the police showed up. I think he wanted to remain alive so that he can have his 15 min. in court and "EXPLAIN" his extremist views. I don't think he should be given his pulpit.