The parent had come to drop their child off at the school in Vitry-sur-Seine, which is being used as a leisure centre during the holidays, police said.
Elite police Raid unit officers and a negotiator are at the scene.
Many schools run summer activities during the holidays.
The school is in the centre of the town, itself some 7.5 km (5 miles) from the centre of Paris.
Police were called at about 07:00 (05:00 GMT) to the school, where 5-10 children and three parents were at the time, French radio reports.
The children were quickly released, said a reporter for France Info radio who is at the scene.
The hostage-taker is said to be in his 30s and unknown to the police. He was wearing a binman's green boiler suit when he entered the school, police sources told French radio.
Although his demands are unclear, he is said to be very determined, police say.
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