Claire Burt, 41, from Devon, said her 15-year-old daughter Lauren was told she would face isolation on Monday 19 September if she did not change her trousers.
But Burt said Lauren wore the same New Look trousers last term and following the comments her daughter was exhibiting worrying concerns about her weight.
“Lauren had the exact same trousers last year,” Burt said. “How can trousers affect her education?
“She keeps asking ‘Am I getting fat?’”.
Burt said Lauren and a number of other students were taken out of classes at Sir John Hunt Community College, in Plymouth, and Lauren was told her trousers were “too tight” and breached the school’s uniform policy and was advised to change by the Monday morning or face further punishment.
Burt claimed Lauren was advised to wear new trousers by the following Monday morning or face further punishment, which could mean isolation or the “sin bin” - studying in school hours but outside of usual lessons and mixed with other year groups.
Burt is refusing to buy a new uniform for Lauren and said the school is to blame for not being clearer in their uniform guidelines on their website.
Burt admitted she didn’t feel she needed to check the website for school uniform policy this year as she purchased the same trousers for Lauren last year.
“I am not changing them under any circumstances,” she said. “It is not about the cost of the £15 trousers but the principle.
“They were fit for school last year, so why not this year? They [the school] are telling us different things.
“I didn’t check before I bought Lauren’s uniform as it [the uniform] was fine for last term and the school said nothing.
“I have looked since and it doesn’t say anything about trousers which are too tight. And in my opinion the trousers are reasonable.
“If there are rules they should be absolutely clear. If they had said in the policy then I’d have been fine.
“But I have told Lauren, if they put her in the sin bin then she is to come home immediately.”
George Perrens, deputy vice principal at Sir John Hunt said: “Uniform is important to us as a college. It allows our students to have a sense of pride, self-respect and confidence in the way that they present themselves.
“During last academic year we felt that a small number of students were not meeting our high uniform expectations, so instead of taking immediate action we elected to ensure that parents were given ample notice to correct their children’s uniform to reduce the financial impact for them.
“Over the last few months we have had an increasing number of students who have not been able to adhere to our uniform policy and have been attending in inappropriate skirts and trousers.
“Our uniform expectations were included in our weekly electronic newsletter throughout June with picture examples of expectations and hyperlinks to appropriate trousers.”