See scalability (Wiki)
scalability is the ability of a system, network, or process to handle a growing amount of work in a capable manner or its ability to be enlarged to accommodate that growth.
In the given cases,
tools that could help make teachers better, at scale
implies those that could measure up to the size of the task.
people who can do great things at scale.
similarly implies people who can do great things when those things manifest in a real-life (larger) systems. It is easier to demonstrate a prototype supersonic aircraft, but extremely complex to build a really usable/ commercial product of the same design concept.
We do not say on a large scale in this context, but instead we say at scale, because the magnitude could either increase or decrease from time to time. Adapting accordingly and producing comparable results in all cases is a challenge in itself.