The rules are to be tightened following the expulsion of 6 pupils and the suspension of 22 others from the school in the southern Swedish town.
When leftover fireworks from the recent new year's celebrations exploded inside the school on Thursday, health and safety representative Leif Paulsson ordered the school's immediate closure.
The incident followed reports of boys urinating in girls' shoes, as well as repeated bullying and beatings on school premises.
The only exception to the new rule is the teaching of foreign languages. Otherwise only Swedish will be tolerated.
"This means that pupils may speak only Swedish in the classroom and in the corridors. This applies even when they are speaking to each other.
"We are doing this so that others will not be able to think that they are saying anything insulting," principal Patrik Helgesson told newspaper Helsingborgs Dagblad.
The Swedish Children's Ombudsman, Lena Nyberg, is deeply critical of the measures.
"As an adult one must try to reach agreement with the children at a school before introducing this type of regulation. Disciplinary rules always work best where there is consensus," Nyberg told Helsingborgs Dagblad.
"The situation in Landskrona seems to be one of abdication, whereby staff have long since handed over power. Now they are trying to take the power back by settinmg the rules unilaterally.
"There is a large dose of discrimination against children who speak a different native language. The first thing that strikes me is that there are many newly arrived children of refugees in Landskrona who have not yet had the time to learn Swedish. Are they supposed to avoid saying anything at all during the school day?" she added.
Parents will be notified if a pupil revert to speaking his or her first languages. If the pupil continues doing so despite initial warnings the parents will be brought to the school for further discussion. If this does not solve the problem all parties involved will be called to a conference to discuss a potential solution.
Gustav Adolf School is the first to impose the sort of language ban suggested recently by two Liberal Party politicians in the nearby city of Malmö.
"Some pupils at Gustaf Adolf School have been threatened and harassed in the native languages of other pupils, which is why the rules have been tightened," said administrative manager Ylva Runnström.
"Only Swedish may be spoken on school grounds. There are already rules prohibiting harassment and this is a continuation of those.
"I am going to raise the issue of whether this should also be the case in other schools. It is important that we have the same levels of tolerance in all our schools," she added.
Ingegärd Milborn, a legal expert at the Swedish National Agency for Education, has not heard of similar measures at any other school and is uncertain as to the legality of the new regulation.
"The question is whether this can viewed as insulting towards the children. Forbidding children from speaking their native language is a sensitive matter. The situation is complicated for the school," she told Helsingborgs Dagblad.