Peter was one of the first teachers in his country___disabled students．．．

to encourage
be the first/second to do sth...是固定用法。

动词不定式做定语（目的状语），to encourage是动词不定式。
同样，动名词也可以做定语，也就是encouraging也可以。

1楼的我有异议：如果是1楼的句型，应该为Peter was the first to encourage the disabled students to pursue secondary education.
这样不能加teacher，所以不是这个用法。

这个句型：Peter是主语、was是系动词、teachers是表语、one of the first是teachers的定语、in his country是地点状语、to encourage the disabled students to pursue secondary education.是目的状语。

be the first/second to do sth...
表示第一个去做什么事的人！