so should I upgrade the hard drive to SSD or it's not worth it for high school students?
If the macbook still works, I would upgrade it to a SSD if cleaning or a fresh install doesn't speed things up. They are relatively cheap now, so even a 256GB SSD is more than sufficient. It will significantly boost load time and make basic applications load nearly instantly.
When I was in high school, a high schooler doesn't need anything that is the latest or greatest, so that Macbook is more than sufficient. They just need basic word and powerpoint editing. They wouldn't care anyway, because once they are done with homework, they'll just go outside and play.
By the time they get into college, their laptops will be more involved, so they may like to get a new machine when they enter college. Save the money for when he is in college.
The decision isn't necessarily about social-economics, but rather being wise with money. It doesn't matter if the parents are millionaires. If you just hand them new toys all the time, they become dependent and it becomes engrained into them, expecting things from their parents.