A posh Upper East Side daycare and preschool that costs up to $40,000 a year is under investigation in connection to an allegation that a student was sexually abused by a staff member — and livid parents claim the administration kept them in the dark for more than a week. The city Health Department was tipped off to the horrific accusation and launched the probe on April 27 into the Second Avenue location of Manhattan Schoolhouse after inspectors found that the staff failed to report the horrific accusation within 24 hours, as required by law, according to agency records. Manhattan Schoolhouse is under investigation after city investigators found the school’s leadership failed to report an alleged child abuse incident in a timely manner.…