The latest Welsh Government figures suggest the attainment gap widened again at GCSE level.
National Deaf Children's Society Cymru is calling for more support and awareness in classrooms. Ministers said they were raising educational standards and investing in pupils with additional learning needs.
The attainment results for deaf children in the last three years fluctuated, according to figures obtained by BBC Wales. In 2014, 48% of deaf children achieved a grade A* to C in the core subjects at key stage four - English/Welsh, maths and science - compared to 64% of peers who can hear. The following year the attainment gap narrowed - but last year it grew again with 48.5% of deaf pupils achieving the grades, compared to 69.5% of hearing children. And the problem is not confined to Year 11.
Over the last three years, the attainment gap has remained the same or worsened at foundation phase and for primary school children aged 7 to 11. The figures come as organisations across the UK mark Deaf Awareness Week, which runs from 15 to 21 May.
Four years ago, the National Deaf Children's Society Cymru launched the Close the Gap petition following a poor set of results. Debbie Thomas, policy and campaigns officer, said the latest results were "unacceptable". "Deafness is not a learning disability so that gap shouldn't be there and we need to make sure that deaf children and young people are appropriately supported so they can reach their full potential," she said.
"There's no reason why they should be underachieving - other than the fact they're not accessing the appropriate support."