Millions of people in Houston were ordered to boil their water before drinking it on Sunday night after a power outage at a water purification plant caused pressure to drop. The mandatory boil-water notice caused public school officials on Monday to cancel classes for at least one day. Authorities say they had not received reports of anyone getting sick from drinking the water in Texas’ largest city but said they were issuing the boil-water notice to be cautious. “We believe the water is safe,” the mayor’s office said in a statement late Sunday. “But based on regulatory requirements, we are obligated to issue a boil-water notice.”