Last week, Adams' Grammar School in Newport announced it will become Haberdashers' Adams from January.
The school, which is one of the oldest in Shropshire, says it wanted to better reflect its "historic links" with the Haberdashers' Company, an ancient merchant guild of London.
But now, an online campaign calling for the move to be stopped has been backed by more than 400 people.
Adams' Grammar School claims that there are only between 120 and 150 "real” names on the petition.
Among them is Arthur Lewis, who said: "As an old boy of the school from 1958 to 1965, I have been proud of my grammar school education which has stood me in good stead throughout my professional career and the name of the school is revered nationally and abroad. It is important to the future of the school that the name is not changed."
In total, 488 people have signed the petition – of which, 100 or so are by anonymous supporters.
The school added a number of those signatures are from people who are not at – and have never been at – the school.
Headmaster Gary Hickey said: "There has been mixed reaction to the new name. Some people love it and are completely in support of it while others are upset by the change; one pupil started a petition against it.
"Some people feel they should have been consulted beforehand but I can confirm that all of the school’s governors and trustees have been involved and are fully supportive of the change.
"I would like to reassure everyone that we will continue to be a selective state grammar school and that this has not been an expensive rebranding exercise as we have deliberately timed it to tie in with normal print-runs and refreshing of the school prospectus and website.”
Pupils were told about the name change in an assembly on Friday morning, which included information about the links between school founder William Adams and the guild. The current crest used by the school will be replaced by the Haberdashers' crest.
The name Adams’ Grammar School has been in use from 1953.
The person behind the petition, who has remained anonymous, said the petition will be presented to Mr Hickey once it has reached 600 signatures.
It read: "While this is intended to reconnect the school with its history with the Worshipful Company of Haberdashers, it will disadvantage future leavers as they will no longer have the benefits of having been to a grammar school on their CV or other applications.
"The school already has enough connection with the Haberdashers with the year seven annual trip to Habs Hall and the annual visit from the Haberdashers to the school.
"The acronym "AGS" is so much more than a name to the students of the school. It is a symbol of great memories and a great education. It cannot just be changed at a moment's notice without consultation with the students and staff of the school, many of whom are not happy with the change."
The petition can be seen at www.ipetitions.com/petition/revert-adams-grammars-name-change