Parents held a protest after an extra class of 30 pupils was imposed on their school.
Mums and dads at Hove's Davigdor Infant School said the move would lead to overcrowding after hearing of the plans yesterday morning.
They also accused Brighton and Hove City Council of not consulting them over the so-called “bulge class”.
Despite their protests, and 200 signature petition, the new class will be in place from September, taking the school's population to 420.
It comes just two years after a similar bulge class was introduced.
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Parent Ruth Cobb, who has had three children at the school, said: “We were given assurances that this would not happen again and there has been no consultation.
“It is a nightmare at the school already with the amount of students and with more students we will face overcrowding.”
In 2012 the council told The Argus the bulge class would be a “one-off” measure.
A council spokeswoman yesterday confirmed Saltdean Primary will also be taking an extra class from September.
The announcement comes a week after councillors warned the city was heading for a school place crisis as early as 2017.
The council said they “regret” having to impose the new class but added it was necessary.
A spokesman said: “We recognise the challenges that this will put on the school, and we are very grateful to the school for its assurance that it will work with us on this and do everything it can to ensure that the outstanding character of the school will be maintained.
“Through this action an extra 30 young children will be able to go to a school that is near to them, rather than having to travel considerable distances across the city.
“We have no information to suggest that pupil safety might be compromised by the extra 'bulge' class.”
She added: “Rising primary pupil numbers is a national phenomenon. Local authorities all over the country have had to create bulge classes in recent years and this is not because of poor planning.
“In Brighton & Hove we have had to create bulge classes in five of the last seven years.”