Those started appearing in the late 70's. My kindergarten class was in one of those.

Those “temporary” buildings were at my middle and high schools for decades.

I graduated highschool in 2010. During my time at highschool we had more and more trailers, but by the end of it (and maybe before that) we had as many rooms outside as we did inside.

I have a strange nostalgia for them. I live in the southeast US, so it's really hot. The AC on these things was crazy. You basically had to wear jeans and a hoodie year round because you'd get too cold otherwise.

The baby boom brought a flood of new school construction. Then as the kid population declined many were turned into administration buildings, apartments, or just torn down to sell the land. Then the existing schools started to fall apart, requiring new construction levies that were hard to pass - "Why should I pay good money to edjumacate somebody else's snot-nosed kids when mine are all growed up!" On top of that, people were moving around a lot more often, so enrollments were hard to predict. Portables became the cheapest solution to make schools flexi-size.

We had these at the COMMUNITY COLLEGE I attended in 2001. Took a math class out there. I say "out there" because they stuck these things far out past the parking lot no one used, beyond the field where they had a golf class. Dark wood paneled walls, the thud, thud, thud of walking on the elevated floors which I'm pretty sure were warped. Awful.

You were lucky if your class was in the trailer because it was the only part of the school that had air conditioning

Yep and this was my school 💯

Our school called it "the annex"

The portables had AC, everyone wanted to go to class in the portables.

There are air conditioning units on those. The ones I was in in the 70s had none. Just louvers the teacher wanted kept closed because they 'interfered with the breeze from the fan' on his desk. The one facing him. The only fan in the room.

is this an american thing?

At least in the US, Reagan lowered taxes and started cutting funding for public schools. A new building wasn't in the budget for majority of school districts.

And they are still there

These are still used. Dumbest I've seen is a new school with them.

It's not great but mobile classrooms are very good when you need to expand but don't have enough demand for classrooms to fill an expansion of say 5-10 classrooms.

Obviously you want to build more than one at a time but building more than you need is not budget friendly. So aiming for 3 mobile classrooms and starting a 5y construction immediately makes a lot of sense.

Keeping these things permanently is just weird.

Had these in the UK too. Bonus points, we lost a huge chunk of playground/ netball courts in the process.

i only knew about these because of malcolm in the middle

They sucked and were a sign of poor planning, but still better than some of the newer schools I've seen as a parent, where it feels more like a prison.

It basically comes down to this

I did 6th grade in one of those. It was actually pretty nice.

Before they rebuilt it in its entirety, the neighbouring school did something similar. The notification to us neighbours listed the construction as a pavilion...

I had family teach at a school that was a series of trailers, but it was after Hurricane Katrina. The lot for recess was basically an 1/8 acre of grass full of fire ants. I have no idea how those kids (and teachers) stayed sane.

meanwhile the student numbers at my former school dropped so much that they started teaching K-1-2 together.