If this is a homework (practice) type activity where students can submit multiple times do you possibly have the show correct answer checked?
I think that we have figured it out. It is on the "Assess It's". The teacher will need to change the "Review" setting to "Never". They are opening 2 windows and copy and paste the correct answer from the first attempt.
It is also possible that if your school purchased this content (which would be used by students across the country) that the answers could be posted on the internet by others students. Try copying the text from the question and pasting it into a search engine.
Yes changing the review settings will help. Also setting a time limit, limiting the number of attempts and password protecting it will help prevent students from seeing the content ahead of time or have more time to hunt for information through another tab.