Use a fleece or sweatshirt that currently fits student and measure from armpit to armpit for measurement WIDTH and from top of shoulder to bottom hem for measurement LENGTH. Compare it to measurements below for approximate size.

XS - Width: 45cm Length: 68cm
S - Width: 50cm Length: 68cm
M - Width: 55cm Length: 70cm
L - Width: 60cm Length: 72cm
XL - Width: 65cm Length: 72cm
2XL - Width: 68cm Length: 74cm

Regatta Fleece are mens sizes, with a raglan sleeve for a more unisex fit.