Tests show the lead levels at St. Vincent de Paul Catholic School, on Fermanagh Avenue, near Roncesvalles Avenue and Queen Street West, are higher than they should be.
On Wednesday parents were informed that lead in the water exceeds acceptable standards..
The lead standard in drinking water is 10 micrograms per litre.
The school found a little more than 14.3 micrograms.
The school is supplying bottled water.
"There are two possible sources," said Corrado Maltese, spokesperson for the Toronto Catholic District School Board, "either within the school lead in is found in solder, or in fixtures, or outside the school [in] city pipes."
It's mandatory for all Ontario schools to do daily flushing of their water systems and annual testing.
The Catholic board has tested most of its 200 schools — and currently 10 are on bottle water advisories.
If exposure exceeds 40 micrograms people may need treatment.
Officials believe the water at St. Vincent De Paul should be safe to drink again in a few weeks.