From your Soundtrap profile, go to the Education tab (1). Scroll down and start a free 30-day trial for 50 users (Starter Pack), or for any number of users (Custom Deal). If you want to, you can start a free trial for your entire school! When you're done selecting, click "Start 30-day free trial" (2) and follow the on-screen instructions to get started!
(1) Profile page (click the Education tab to open it)
(2) Education tab (scroll down to see this view)