I would say there are hundreds or thousands of these discoveries waiting to be made throughout the world.

The end of the article notes that it is commonplace for altars to embed relics of a saint, especially a martyr. It has typically been considered mandatory through history and the Second Vatican Council. In fact, a custom developed of using an "altar stone" which would permanently hold the relics, and serve as a portable place to celebrate Mass, if the immovable altar wasn't sufficient for that.

The thing is, with Vatican II many, many parish renovations included the addition of a freestanding "table altar" which was often new construction. This would often mean that the "high altar" of sacrifice would become disused, de-emphasized and sometimes just pulled out of the wall and placed in storage.

So what they found in this school is apparently a disused "high altar", no longer in service, and forgotten in storage.

So, not surprising it contained relics, but exciting that the documentation is intact. Indeed, the practice of certifying and documenting relics is important, because who wants a fake, or a stolen one? How many burial cloths of Jesus, how many nails are there? You could build a cargo ship with all the True Cross wood in Europe.