Does your school have a sheet metal bender? If so go for some 2mm steel and you'll be fine. If by hand then yeah around the 1.5mm mark (although I have bent 2mm titanium by hand )

As shane said, go for thicker than 0.9mm. Are you able to stick a box section bumper on the front? Rather than thickening up the material, I would look at modifying the design. Bolt a piece of 20mm x 20mm with a 2mm wall thickness mild steel box section and it will be fine.