Assuming this statement is true (do you have a source) there’s multiple things going on here.

1. A decline in the birth rate does not necessarily mean a decreasing population.

First population change is given by:

Total population - deaths + births.

As long as births > deaths your population will grow.

For example, let’s say in 2020 you had a population of 1,000, a death rate of 10% and a birth rate of 20%. That would give you:

1,000 - 100 + 200 = 1,100 people. A growing population.

Let’s look at the next year. For simplicity sake 100 people moved away so our population is back to 1,000. Death rate stays the same, 10%, but birth rate declined from 20% to 16% (a 20% decrease).

Your population equation is:

1,000 - 100 + 160 = 1,060.

Not growing as fast but still growing.

2. Even if you had an overall declining population, that doesn’t mean the decline is the same everywhere. Some areas could be declining more quickly while others are declining more slowly or even increasing.

3. People move. Even an overall declining population doesn’t mean a particular area will become less crowded if more people are moving there.

4. It takes time for changes in population growth rates to be felt. Especially if there isn’t a consistent change over time. If that 20% decline is replaced in a few years by 20% increase you’ll see things even out over time. Aside from your kids class being more or less crowded for a few years you might not notice the change at all.