The phrase "an additional classroom" is correct and usable in written English.
It can be used when referring to an extra classroom that is being added to an existing number of classrooms, often in the context of education or facility planning.
Example: "Due to the increase in student enrollment, the school decided to build an additional classroom to accommodate the growing number of students."
Alternatives: "an extra classroom" or "a supplementary classroom".
I'm going out to help refurbish a school and build an additional classroom for kids that have it tough.
Kwa Njenga Primary School, North Berwick, East Lothian: support an associated school in Kenya with funds toward an additional classroom.
They built an additional classroom after the first classroom was met with positive reactions.
In the mid-1960s, additionalnal classroom wing was added, followed in the early 1970s by construction of a vocational classroom wing and the auditorium and adjoining scene shop.
When the delegation visited back after the specified period they noticed the change of the community's attitude towards education: additional 39 girls and 18 boys were admitted to the school; an additional classroom and a fence for the school were constructed with local materials.
If you are already a teacher in Texas in another subject area and wish to become certified as a bilingual teacher all that is necessary is an additional classroom certification exam in the ESL subject area and the specific language subject area.
And as a short-term solution, the city had named 26 Broadway as an option for additional classroom space.
The North Academic Building will provide an additional 858 classroom seats through a variety of classroom sizes and styles.
The $64 million project — financed partly by money from the museum's endowment — will create at least 60 percent more exhibition gallery space, a new library and additional classroom space for the museum's master's program in decorative arts and design.
We have also demonstrated that flipped-class pedagogy can be used in combination with a traditional class without additional classroom or staff requirements.
The expansion will include a library media center and science lab in each school along with additional classroom space to ease a situation where there has been an average of 25 pupils from kindergarten to grade 3.
