GameMaker Studio 2 is a free game engine that is easy to use.
We use it at QE2 in Year 9 as well as for the GCSE Digital Technology Course.
You can download the software from the GameMaker website.
YouTube has hundreds of tutorials explaining how to create games with GameMaker:
GameMaker tutorial from the makers of GameMaker
Make a Maze Game by Newport High ICT Department
Make a Platform Game by Sky Anderson