Cuts to teaching assistant posts are putting more pressure on teachers, a union has warned.
Official figures showed 23,994 support staff were employed by schools across Wales in 2016 but that was 446 fewer compared to 12 months earlier.
The number of teachers is also down by 275 to 23,235.
But the Welsh Government said the pupil-teacher ratio has remained "relatively" stable.
"After years of cuts, head teachers have little wriggle room but to look at the staffing they have," Owen Hathway, the National Union of Teachers policy officer in Wales told Welsh language news programme Newyddion 9.
"This needs to be avoided because any teacher or classroom assistant that is lost puts more pressure on that staff who are still there.
"That isn't fair on the children when we're trying to ensure they get the best education in Wales."
Jess Turner, from Unison which represents ancillary staff in schools, said schools needed more than just teachers to run properly.
She added: "Every time a caretaker, cleaner or admin worker loses their job it means more pressure on classroom staff.
"Those who remain do their best to keep up standards in their schools but with fewer staff, safety and security is bound to be compromised."
Tegwen Ellis, head teacher at Ysgol Cynwyd Sant in Maesteg, said accessing more money was essential in order to reduce pressure.
Her school's budget is £70,000 less in this academic year - a situation not uncommon across Wales - but they have managed to bridge that gap by securing further grants from elsewhere.
"Some schools get sponsorship from businesses, some schools are working alongside businesses to develop children in a different way, and we as a school have to think in a different way as well," she said.
A Welsh Government spokesman said: "We are well aware of the financial challenges facing our schools and have prioritised funding for schools within the settlement we provide for local government in Wales.
"Over the past five years to 2016, there has been a reduction in teacher numbers but the pupil-teacher ratio has remained relatively stable (18.4 to 18.6)."
The spokesman added there were 1,700 more support staff than five years ago.