What is the flow rate of shielding gas for different steel thicknesses?

I presume your talking about MIG, and not TIG. I run about 7 litre/min, for the lighter (up to 1.5mm), and turn it up a little if your welding upside down, as the gas is heavier than air. You can run up to 12 l/m on heavier steel as the heat will take a little longer to dissapate. On saying that, a really good welder will take 2 passes on 5 mm steel. You can use 75% Argon/25% CO2 on very light steel for a better bead and less spatter.

I am a self taught welder with 20 years experience, using MIG and TIG, and I am very proud of my welding. I build race and track cars, including in-house designed suspension components in T45 (EN14), and roll-cages. I have never seen flow rates quoted for MIG, but if you google "Miller weld Calculator" it may give you some guidance, I used it when I bought my TIG.

I am not a teacher, but you may find your teacher cannot weld. I got some guidance a few years ago from a "welding instructor in a 3rd level school, as I was preparing for a weld test, He was useless.

John