The quality of teaching in Wales is weak, according to the education watchdog Estyn in its annual report.
It was the "weakest aspect" of provision across most areas of education in Wales, its chief inspector Meilyr Rowlands said.
Teaching was "good" or "better" in only a minority of secondary schools inspected this year.
Education Secretary Kirsty Williams said improving teaching standards was "one of my highest priorities".
Mr Rowlands said too few schools help staff to make the best of professional learning opportunities.
The 157-page report also found in about a third of primary schools inspected this year, more able children underachieved because their work was not challenging enough.
Mr Rowlands also said school heads needed to create better opportunities for teachers to develop their professional skills.
"What needs to be done to raise standards in education in Wales is becoming clearer and improving teaching will have a long-term effect on quality and standards in the classroom," he said.
"The best teachers have high expectations, challenge their pupils and think critically about their own practice.
"Leaders in education need to have a strong focus on providing suitable opportunities for the professional development of staff at all levels in order to nurture confident and creative teaching and learning."
Mr Rowlands said improving professional learning and collaboration between schools could "drive out the variability that still exists in our education system".
However, there is some good news for primary schools in Wales.
The report said younger children's literacy and numeracy skills are improving. Standards were good or better in about seven in 10 primary schools inspected.
There was also concern about secondary schools.
As well as finding teaching "good" or "better" in only a minority of schools, the report found secondary schools continued to have greater variability than primary schools with "more excellence but also more unsatisfactory practice".
And when it comes to the brightest children, Estyn said schools' expectations of their more able pupils just are not high enough.
'Deficit of strong leadership'
Conservative education spokesman Darren Millar AM said the report "highlights a huge deficit of strong leadership in around half of schools across Wales, which is holding back teachers and children from achieving their potential".
But Rob Williams, policy officer with head teachers' union NAHT Cymru said the responsibility lay across the whole education system.
"Sufficient and fairly distributed school funding has long been required to enable school leaders to deliver the investment that the workforce need and deserve," he said.
"There also needs to be a recognition that in order to focus on developing great teaching, pressures need to be eased elsewhere - this requires prioritising and doing the most important things well."
NUT Cymru secretary David Evans added that there were "significant barriers" to promoting better collaboration between schools or better access to professional development "from a lack of high quality training provision, a lack of financial resources to release teachers or workload pressures making non-classroom activity almost impossible".
Ms Williams said: "This report provides us with valuable evidence about performance in our schools which can help us continue to drive up standards in education.
"We will study the report in more detail before formally responding in March."
The minister also pointed to the recent announcement of a new National Academy for Educational Leadership in Wales and launching new professional standards as examples of reforms.
"No one can accuse us of sitting back and accepting the status quo," said Ms Williams.
"I remain committed to the ambition of providing all teachers and leaders with the right skills and knowledge to benefit pupils and make sure schools can deliver the new curriculum and vision for education in Wales."