How to limit a student's access period to the group in the Members Area?

By default, users signing up for the Members Area get unlimited access to the content in the group. To specify a time limit for accessing information, click on the Group Settings in the right top corner. 

In the Group Settings, specify the access type: Unlimited access, For the period, or Until the date selected.

In the settings, you can select the "Apply for all members" checkbox — if you use the option,  a new rule of limited access will be applied for both existing and future members of the group. If you do not use the option, the limited access rule will be applied just for new members.

Note that if you select the "For the period" option, the time for existing students will be counted from the moment you have configured the settings and saved the changes. For new members, the period will start from the moment they have signed up for the group successfully.