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DENVER (KDVR) — Colorado’s intense September heat is having an effect on students across the metro area who attend classes in older buildings without air conditioning.

On Thursday, Denver Public Schools closed three schools and dismissed class early in eight others.

Upgrading older buildings can be a challenge.

Denver voters passed a bond last year to put air conditioning in 55 schools, and 19 are scheduled to receive the upgrades over the next two summers.

Parents are concerned about students being exposed to excessive heat.

“It’s very hot outside, and they need to be cool,” one parent told FOX31.

Teachers agree that a healthy learning environment is free of distraction and discomfort.

“They become dull, and when you ask them what’s wrong, they’ll tell you it’s hot,” Grant Beacon Middle School Principal Jennifer Anderson said

Anderson said the heat “adds a layer to our teaching staff which causes fatigue and burn out.”

DPS said the problem with air conditioning upgrades is that many schools were built in the late 1800s.

“One school just turned 100 years old. Trying to modify a school that was designed then to meet today’s needs and requirements is very difficult,” DPS Director of Construction Heather Bock said.

DPS installed an energy-efficient HVAC system at Grant Beacon Middle School, which uses the existing ventilation structure. The installation took less than three months during the summer break.

DPS spokesperson Will Jones said that when considering the design of the facility or the age of the building, each school is unique.

“One of the ways we work to support our students is by providing a learning environment that provides comfort, proper temperatures and a place for them to feel safe. By doing so, we are helping our students to thrive,” Jones said in a statement.

Here is a list of the schools impacted by early dismissals.