sorry for the delay getting back to you. what you're describing is a known issue, and something that's on the list of things to fix. in the meantime there are a couple of options:
1) have your student create a second account that is used for your discoveries class. (i realize this is not ideal)
2) have your student stay enrolled in your CSP section, but access the discoveries activities directly via url (for example, access unit 1 via this link: https://studio.code.org/s/csd1). you will still be able to see the student's work on your teacher dashboard, where you will need to navigate to whichever section "student A" is in, then look at progress in the cs discoveries unit of choice. (as shown below)
hope this helps!