working on excel files through website

Hey...
I work as a physical therapist in a school for disabled children.
I set up an excel sheet so teachers can enter points in the childrens
monthly report.
Now, they all have to sit on the schools computer to enter the childrens
results... this gives a lot of problems and is difficult to organize.
I have set up a website (very basic but it works...)
Now I am asked to figure out if the teachers can't enter their points at
home via the internet.
I know I can put xls files on the net, but i don't know how to set up a
system so teachers can down, and upload the xls files AND... what if 2
teachers work on the same file...
I am not a specialist so keep it simple ok???? thanks a lot
JP