Can I ask students for proof of a negative COVID-19 test before they enter my lab or studio?
You should not ask someone for test results. You can communicate to students that your expectation is that they have completed their required testing and that they are following isolation requirements if they tested positive.
No one should come to a lab/studio/work/class if they have symptoms or have tested positive for COVID-19 until the isolation period is completed and symptoms resolve fully. If someone tests negative, they can return to a lab/studio/work/class while continuing to abide by all safety procedures and protocols, including masking, distancing and hygiene practices.