For beginning band students, I strongly encourage renting an instrument vs. buying. Here are some reasons why:
Students are learning to care for the instrument and usually rentals can be repaired by the shop for no cost.
Students can end up switching instruments in or after their first year.
It is tempting to look for the cheapest instrument you can find; the quality of a rental is FAR better.
Lower quality instruments make it more difficult to play and cause frustration, lowering enthusiasm for learning.
Shop costs to repair inferior instruments can quickly exceed the cost of the instrument and some shops won't work on them because they cannot guarantee that the repair will not fail.