One distinction may be that the star teacher scales better. What if there were two students with this problem? Should the teacher stay late? Should teachers not take lunch? What if someone needs help in math, etc.
A star teacher may be able to reproduce this effect for a large number of students. Perhaps using technology, recorded lectures, practice apps, etc. Perhaps organizing study groups for kids to teach and learn from each other.
A caring teacher may sacrifice her lunch. A star teacher may not need to.