Two guards took up their posts at the two sites occupied by Danemill primary in Enderby on Monday morning.
They have been provided by Kevin Chapman, who runs the Elite Security company, after full consultation with Danemill's acting headteacher, Jo Elsey, and the school's senior teachers and governors.
Mrs Elsey said: "It's a very generous offer and I think parents will be pleased we have taken it up.
"Parents must be wondering what their children are going to each day when they go to school now, and I think our parents will rest a lot more easily now they know someone will be there. The school could not have afforded this if Mr Chapman had not volunteered the service."
Mr Chapman, whose seven-year-old son, Sebastian, attends Danemill, said: "This problem is out there and it's going to get worse."
Parents were told about the introduction of the security guards by letter.
The guards will be on duty throughout every school day, and Mr Chapman estimates that the service will cost him around Pounds 1,300 per month.
Nevertheless, he hopes that the arrangement will become permanent. Mr Chapman and the school authorities will meet to evaluate the situation in four weeks' time.
Stuart Lindeman, Leicestershire's deputy education director, said that schools would have to balance the need for children's safety and security with the need to remain welcoming to the local community.