First you have to press 'Download' button on the game site. Then press right mouse button on the downloaded file and press extract. After that find a folder named the same as file that you just downloaded so it will be something like 'School Simulator Pre-Alpha 0.4' (it depends on the version that you downloaded), open it and find file 'School Simulator'. Open it by double clicking and press play. I hope that it will help you :)