Teaching at Grays Convent High School is being disrupted due to ongoing industrial action.
The five main teaching unions in Northern Ireland are to ballot their members from 10 June.
Union members have taken 13 days of industrial action at St Cuthbert's RC High School since February,
Parents voice support for striking staff at Queen Elizabeth's School and Corfe Hills School.
Banovallum School and Queen Elizabeth's Grammar School in Horncastle will be affected.
Staff members at Queen Elizabeth's School and Corfe Hills School took industrial action on Wednesday.
The National Education Union announces plans to strike at Dorset schools in May.
Teaching staff will walk out for three days over the academy trust's plan to cut staff and subjects.
NASUWT's leader said there is a "ticking time bomb" if male pupils cannot be helped with misogyny.
