By default you only see the students who created their account in the current academic year. If a student created an account in a previous year you can transfer them to the current year so you can always see them immediately. 

For that you'll need to change the year in the upper right corner. A list of past academic years will appear and you can select the one you want. 

You can also change the academic year of your students individually by clicking on "Change season" behind the name of the student. You can also select several students at once by clicking in the boxes on the left sid of their names. Now the button "Change season" at the top of the page will be able to select and you can move the selcted students to the season you want.

That way you can acces their current statistics and latest game activities quicker.

Another way is to create a class if you like - you'll find the instructions here.