At that age safety is mostly about education and knowledge rather than where they are. It's a great suggestion above to have somewhere that doesn't need them to walk on the road to get back.
They need to understand what makes a holiday unsafe, and almost everything is within their control. The risks are mostly alcohol, combined with traffic and hot sun. They must know not to sleep on the beach and risk lying in full sun as it moves around, they must know not to walk on roads with too much alcohol, not to swim in sea out of their depth after drinking, not to dive into swimming pools, not to fool around on a balcony, not to go out with more cash than they need for that trip, not to hire a moped.
it's a good idea if they get together and make a group agreement - for example no-one goes off without the others at night however appealing that boy seems to be, anyone going out alone in the day takes a mobile phone and leaves details of approximately when they will get back. It's a good idea to have a nominated non-drinker at all times.
They should have a great trip. One thing to remember is that opinions on accomodation reviews usually tell you more about the poster than the accomodation, stick to facts. A group trip like this is a great start to adult life, have fun, enjoy the planning and relax.
PS Mine are 18 and 16, both starting to reach out and enjoy trips with friends