It is not illegal, but it is not advocated. There is nothing wrong with maintaining your nails, but if you do decorative nails, it will affect your impression among colleagues and classmates. Most people will think that you haven't done your job well and put your energy and time into teaching. Manicure is not the same as makeup. Make-up is to improve mental outlook and establish a positive image among students and colleagues. But there are just too many manicures. Especially when your work is flawed, it is easy to be magnified because of your manicure. Teachers are not advised to manicure.