Khat has been branded an "evil drug" by campaigners in Leicester, where it is traditionally used by members of the Somali community, who chew the leaves for its stimulant effects.
The city has a large Somali population, and it is understood Somali children are routinely buying it in independent stores without being questioned.
Educationalists have now joined forces with Somali elders to get the herb which is legal in the UK removed from shelves. "This is an evil drug and children who are taking it are struggling at school because they can't concentrate," said Kayse Maxamed, editor of Somali Voice, a newspaper founded in Leicester.
"You can buy Khat easily in the city and when you take it, it becomes impossible to sleep and you lose appetite. It has a very negative effect on mental health and there are people admitted to hospital because they chew it. It needs to be banned."
Its prevalence made it hard to police, added Bill Morris, spokesman for Leicester's secondary school heads. "Any substance that affects mood or concentration is bound to have a detrimental effect on learning, and unfortunately Khat is widely used," he said.
Leicester NHS has also warned of the herb's dangers, saying its side effects include insomnia, paranoia, mood swings and mouth infections, which can all contribute to poor performance at school.
There was also evidence that it could lead to violence in the home, it said. "The concern is that the plant is in local grocers and children are walking in and buying it," said spokesman Hashim Duale.
Khat is already banned in the US, Canada and some European countries.
A Leicester retailer, however, defended the sale. "Yes, children do come in and buy it, but it is traditionally used in our community," he said.