It's not always like that. During periods of particular security alert (like when we had the bombings on the métro here in Paris) the schools can be ringed with those heavy barriers to prohibit people from parking too close to the school.
In the public schools, there generally aren't receptionists or whatever, and parents aren't particularly welcome inside the schools during the school day. They make the parents wait outside when they are picking up their kids at lunch or after school - even in the pouring rain. The international schools may be different, but my impression is that school security in France is generally pretty good, at least for the younger kids.