And although the 25-year-old IT trainer took the comments in good humour, he was determined to shed some pounds.
Now James is half the man he used to be after going from 25st 11lb to14st 8lb in just two years.
James, who used to gorge on fast food and fizzy drinks, now has a 36 inch waist – down from 60 inches at his biggest. And he has his cheeky pupils to thank for his new slimline look.
He said: "I got some comments from pupils - the word 'tubby' was used a few times.
"I took it in good humour because I have a good relationship with the students but things came to a head when I had my picture taken on sports day.
"My body virtually filled the entire picture, which was embarrassing and I knew I had to do something to change my appearance.
"I looked in the mirror and thought, 'Who is that person?'"
James, who teaches at Oldbury Academy, West Midlands, took up cycling and a healthy diet and has reaped the benefits.
He added: "The weight has managed to keep coming off and now I feel much better.
"When you are used to seeing yourself every day you don't notice the change.
"But I do think it s ridiculous when I look at photos from a few years ago and get my big trousers out."