The government’s “pedantic” new spelling tests for primary school children will stifle creativity and discriminate against pupils with dyslexia, teachers and campaigners have warned.

The British Dyslexia Association (BDA) said it has been inundated with calls from primary headteachers who are alarmed about the new system, which will require 10- and 11-year-olds to correctly spell more than 100 key words before they are judged to have reach expected educational standards. The system will come into effect for exams taking place this summer.

Following an outcry from teaching unions, the government attempted to clarify the new writing assessments this week by partially backtracking on the proposals. But campaigners have argued that the concessions offered do not go far enough.

Russell Hobby, the general secretary of the headteachers’ union, the NAHT, said: “We have significant concerns about the treatment of children with dyslexia: we are worried that there is a risk of discrimination. Schools are prevented from properly recognising the successes of dyslexic students in the way they can with other students.”

Emily Gazzard, an English specialist at a north London primary, said the new system was “draconian, pedantic and stifles creativity 100%”.

She added: “I have a child in my class who has English as an additional language and I think she is probably also slightly dyslexic. She writes beautifully with dialogue, fantastic use of inference, personification and metaphor – but because her spelling is weak she will probably have to go up to secondary school as a child who has not met the expected standard. It is absurd.”

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Sue Flohr, a helpline manager at the BDA said: “We’ve had an unprecedented number of calls on this from teachers and headteachers. Spelling didn’t used to have such significance. Now, if a child writes a beautiful piece of work with the wrong spelling they are not going to pass. And then they will be misplaced in secondary school.

“It’s discriminatory because pupils who fail in English will be disadvantaged.”

Gazzard outlined her concerns in an open letter to the education secretary, Nicky Morgan, which has been shared thousands of times on Facebook.

She wrote: “For many years, I have encouraged children to develop their vocabulary in their writing, urging them to be creative and ambitious with their word choices. Now this ambition must be curbed as teachers will be encouraging their pupils to use only words they can spell correctly … Those children with SEND [special education needs and disabilities] who have flair, creativity and write with a strong authorial voice will be deemed as not ready for secondary. This seems discriminatory.”

Morgan has not responded to the letter but in a statement her department said: “Spelling and handwriting are key elements of the national curriculum in primary school. To enable all children to reach their full potential, it is essential that they develop their skills in these areas, including those with dyslexia.”

The department published a “clarification” on the new assessments this week in response to the alarm expressed by many teachers when the changes were first announced in February. Concessions included allowing pupils to use dictionaries for assessed work and to submit redrafted work after discussions with teachers.

Gazzard said the updated document had “caused more confusion than clarification”. She added: “It doesn’t go far enough. They should move away from the checklist model altogether.

“In the old system, the majority of marks were about composition. That’s just vanished. Now we are having to tell kids to forget about ambitious vocabulary, because it’s all about accuracy. So use ‘bad’ instead of ‘disastrous’, because you can spell it.”