This is an important teachable moment
One of the most annoying traits in newer engineers is their "helpfulness." They may not realize that working on tasks outside of their assigned scope can actually be harmful. Often there are separate team members who are specifically assigned the visual stuff, and sometimes there are strict standards, compliance issues, accessibility issues. When somebody else decides to be "helpful," it can actually cause a huge setback for those specialized developers. Or sometimes the "extra" work was excluded from the contract and giving the client free work can harm sales. Either way, it is a critical soft skill for engineers to learn to "stay in their lane."
This topic is important enough for its own lesson. Let them fail at this on their first assignment, and then use the next class to Tell them this is important. And that they will be marked down if they do it again. If students do extra work anyway, it should result in a decreased grade.
If you feel bad marking them down, you can offer an opportunity to fix their mistake (an extra credit assignment), and in so doing also teache.g. grant them how to do rollextra credit for rolling back sourcethe code to a priorthe original version. Then you can teach them about the importance of source control too. Two teachable moments in one!