This is much more complicated than you think. If people stay for an extra year, how many new scholarships can you bring in? Do you have extra scholarships for 5 years? Do people automatically get a 6thyear to play their 4th season? Depending on the answers this can be brutally expensive.
Also why are the students at any school? Are they there to get a degree and possibly start a masters or are they there to stay "forever" and to play a sport? I am in the degree column. How many of the students need another to show they can go pro? If after 4/5 years they do not know, that 6th year is not going to get them noticed.