Staff have had to close off parts of the playground and school field at St Patrick's RC Primary in Higher Walton near Preston because they are worried railings and hedges are not secure.
Plans have been submitted to South Ribble Council to build a 2.4-metre high anti-climb fence around the entire school site to make it safer for children.
The scheme follows a safeguarding audit last October which highlighted concerns over keeping youngsters in and members of the public out.
The survey found children could squeeze through railings at the front of the school and onto a busy main road.
It also discovered hedging on the perimeter of the school fields at the rear had gaps which were wide enough for youngsters to crawl through and dogs to access the site.
In a statement to the council's planning department, headteacher Liz Hindley said: "The perimeter of the school site is currently not secure, causing significant safeguarding issues for the school.
"Firstly, the school backs onto private properties. Fences at the back of these properties are in various states of disrepair.
"Secondly, the side of the school field is hedged and patchy, which sides onto a public footpath. This is not safe.
"Finally, the iron railings across the front of the school are very low and spaced too wide apart. Our children are able to slip between the gaps, which is not acceptable."
Mrs Hindley said the problems were having an impact on school life.
"We are unable to allow our children to play towards the rear of the school field as it is not safe.
"We have had dogs running across the school field through the hedge.
"This also indicates how easy it would be for the children to crawl through the hedge.
"We cannot allow our children to play in the playground across the front of school as the gaps between the iron railings are big enough to allow a child to pass through.
"This is not safe and also limits the already woeful outdoor space that we currently have for the children to play in."