You can use Hapara to limit students to certain pages/websites.

For the most up to date info, see Hapara's help article on this,
https://support.hapara.com/hc/en-us/articles/213860988-Using-Open-Tabs-Focused-Browsing-and-Schedule-Browsing

Open your class in Hapara and click on the Highlights tab. Then click on the highlighted icon to open a site on your student's computers.






Click on the highlighted icon to enter Focused browsing mode.





Enter in the site(s) you want to open on your student's devices, and configure the options as desired.
You can change the duration of Focused Browsing, and select all students or just a student from the icons on the left.
Once you are ready, click "Open Tabs".
What this will do is, close any tabs the students currently have open, and open the websites you have entered in. If you left "Restore tabs after Release" the tabs the students had open previously will be reopened once the session is complete.





You can end the Focused Browsing session at any time by clicking the highlighted icon, or by clicking close tabs. Close Tabs will close the site you opened on the student's device, so if you want them to keep working, don't use it.






Below is what a student will see. Right before switching to Focused Browsing, they will get a pop up in the bottom right.




After that, the website you entered will open.



If the student tries to go somewhere else, they will get this,