Leeds City Academy, in the Woodhouse area of the city, sent students home at 12.30pm today and will be closed for the whole day tomorrow (Friday).
The academy has experienced an increasing number of staff and students with symptoms of the norovirus since Monday.
Norovirus, also known as the winter vomiting bug, can spread quickly in hospitals and schools because people are in close quarters and children can forget to practice good hygiene.
The main symptoms are feeling sick, being sick and diarrhoea and the bug usually goes away in around two days.
Leeds City Academy is the third school in Leeds to close due to students becoming unwell, with Leeds East Academy and neighbouring Parklands Primary also being forced to close today.
The three schools are set to reopen on Monday, but parents are asked to keep an eye on the school's social media accounts and website for further updates.
In a message to parents and carers, Leeds City Academy said: "Since the start of the week, Leeds City Academy has experienced an increasing number of staff and students who have been affected by symptoms similar to those of the Norovirus.
"Throughout today, the number of staff and students being affected by these symptoms has increased to a level where the appropriate course of action is to close the academy to all staff and students at 12:30pm today.
"The academy will also be closed tomorrow.
"We sincerely apologise for any inconvenience that this decision may cause to our community."
The school has undergone a deep clean throughout the week and will continue to be cleaned while the academy is closed,
The message continued: "This decision has not been taken lightly but as our school community will know, our first priority is the safety and wellbeing of our students and staff."
NHS advice on how to treat diarrhoea and vomiting:
stay at home and get plenty of rest
drink lots of fluids, such as water or squash – take small sips if you feel sick
carry on breast or bottle feeding your baby – if they're being sick, try giving small feeds more often than usual
give babies on formula or solid foods small sips of water between feeds
eat when you feel able to – you don't need to eat or avoid any specific foods
take paracetamol if you're in discomfort – check the leaflet before giving it to your child
do not have fruit juice or fizzy drinks – they can make diarrhoea worse
do not make baby formula weaker – use it at its usual strength
do not give children under 12 medicine to stop diarrhoea
do not give aspirin to children under 16
Stay off school or work until the symptoms have stopped for two days. Also avoid visiting anyone in hospital during this time.