Some pupils are reportedly arriving as early as 7.55am as parents start work at 8am.

While some schools offer before school care, those children not using the service mill in the school yard or outside the gate until official playground supervision begins around 8.30am.

Most primary schools begin classes at 8.55am.

Australian Education Union ACT Secretary Penny Gilmour said she was unaware of how widespread it was but it could occur in pockets across the territory and was a concern.

''There are safety issues around traffic, potential for stranger danger and other things,'' she said.

She understood pressures on parents to get to work on time, but ''teachers don't actually stay at school 24/7'' to look after children.

''Teachers are parents too and would also be organising their own children to either go to before school care or whatever the other appropriate arrangements are,'' she said.

The principal of St Jude's Primary School in Holder, Michael Lowe, said pupils getting to school ''earlier and earlier'' was a troubling issue.

Some of the first pupils started arriving between 7.55am and 8.05am a full hour before the school day began.

Mr Lowe, who usually gets to the school about 7am each morning, said children arriving early were instructed to sit under the shelter shed inside school grounds.

Noticing the number of early arrivals the school had moved its playground supervision time from 8.40am to 8.30am.

Mr Lowe said the school had also attempted to establish a before school program, but when they surveyed parents they did not have enough demand to make it financially viable.

''We often write to parents reminding them that supervision doesn't start until 8.30am,'' he said.

Principal of Red Hill Primary School Chris Hamilton said it also had pupils arrive before school supervision but didn't think the problem had become any worse.