"This is a picture of my teenage son just getting in from school," she wrote.
"Why is he getting in from school at 10.53am you ask? Schools don't finish until 3pm.
"Well the little turd has just been expelled from school for the day after being caught charging students 50p ($1) a squirt for hand sanitiser to protect themselves from the bloody coronavirus!!"
His mother, however, said she tried to be serious when the school called, and later when her son arrived home "with a big grin on his face".
"Very hard to discipline this behaviour when his dad phones him from work to call him a ****ing legend."
She told Daily Mail she thinks the school overreacted.
"I don't think it's an excludable offence. I told him off so that when we go into school, I can tell his head of year that I have told him it's wrong.
"I think they see it as him exploiting a situation, but his stepdad called him a legend halfway through me telling him off so that put a stop to it.
"I won't be punishing him any further."
READ MORE:
• Coronavirus: Angela Merkel warns up to 70 per cent of Germans will get infected
• Coronavirus: No effect on house sales, new national price record
• Coronavirus: What's the difference between a pandemic, an epidemic and an outbreak?
• Coronavirus: World Health Organisation officially declares a pandemic
She said her son was just trying to make a quick buck, revealing he planned to use the £9 profit to buy a kebab.
A school spokesperson told Daily Mail "No student has ever been excluded" for "selling hand sanitiser".
Tompkins' post has been shared more than 164,000 times.
Many readers praised the boy for being safety conscious and entrepreneurial.
"The school should be happy he is using his initiative not expelling him! Well done young man, you will go far," one wrote.
Another said: "Bet he gets an A in economics."
One reminisced about selling cigarettes for $2 a pop back at school.