They suffered serious burns after lightning crashed through Runyanya primary school in the east African country's Midwest about 160 miles west of the capital, Kampala.
The lightning struck at about 4.30pm local time (1.30GMT) as pupils sat in their classrooms waiting for a downpour to stop before setting off home, said police commander Patrick Byaruhanga.
Dr Jimmy Eyiiga, the Kiryandongo medical superintendent confirmed this morning that 18 pupils and one student teacher are now dead. He said other pupils who were not yet out of danger have been sent to Mulago National referral hospital, for specialist medical attention.
Last night, about an hour before the school tragedy, lightning struck a village in Sironko, killing a 58-year-old man during a heavy down pour.
Deaths by lightning strikes then spread to Karamoja, in north eastern Uganda, striking another school in Kotido district, killing another pupil instantly. One pupil died on the spot while another one was left unconscious, said the country’s Daily Monitor newspaper.
In Karamoja, police spokesperson George Obia identified the victim as Raffle Lotyang who was a primary school pupil at Kotido Army Primary School.
Though the country is in its second dry period, which runs from June to August, fears about the number of recent deadly lightning strikes prompted MPs to raise the matter in parliament yesterday.
Lightning has killed up to 28 people and scores injured in the past week, the private Daily Monitor reports.
School buildings are being hit because they don't have lightning conductors and are built on high ground, say weather experts. In the past few weeks, lightning strikes around the country have killed at least 34 people.
The country’s police and fire services today asked people organising events and gatherings to make sure there is lightning fighting equipment or repellents and that the venue’s wiring meets electricity standards.
Uganda Police Fire Brigade commander, Simon Peter Musoke urged schools, places of worship and other buildings that house large numbers of people to have lightning conductors put in. And while the government looks for a solution, he advised that people should at least avoid sitting in big groups to shelter from the rain, to reduce the number of victims if lightning does strike.