block all websites but 3 for 32 students logged into google accounts on chromebooks

i am a teacher in the hood and i have a real problem.they have chromebooks to use in class so they can complete their weekly required work on this specific website. they are required to log on-task 45 mins per week on this website.its my job to make sure they achieve this and it usually takes about 3 hours to achieve this bc they are relentlessly off taskthats 2.25 wasted hours that i could use to cover other material that im also responsible fori need to be able to block all websites besides this one or 2 websites so that they will not have any distractions and complete this work in a timely manner.i found an extension called block site and better yet, white list manager that will do this however i think they may eventually figure out how to turn it off.is there anything that does this but with a password that would be needed to turn it off?they log into chrome also so i also would need them to log in and then pass me the chromebook so i could install the extension and this will definitely be an issue.they will know something is up and try to prevent me from doing this. it will be a whole big thing where i’d need to get administration involved and id rather not need to do that.id just absolutely love for their favorite websites/distractions to be conveniently blocked at my discretiondoes anyone know of a way for me to acheive my goal and perhaps conveniently be able to turn it off and on from a separate computer?i will not get access to their google passwords as far as i know and even if i did, it would be pretty time comsuming for me to log into 32 accounts each day to turn it off and onthank you in advance if you can help me id really appreciate it via /r/tech https://ift.tt/2XwNtHa