Subordinate clauses that are not for comparison - should we use が or は？

Question

In a subordinate clause, が is often used.

However, some sentences with けど or のに are used with は because they are for comparison.

But it seems to me that some sentences with けど or のに are not for comparison.

In those cases, do we use が or は?

Example 1

先生（が/は）答えを教えたけど、弟は低い点数を取った。

My younger brother got a bad grade although the teacher had told him the answers.

Example 2

彼氏（が/は）プレゼントを買って置いたけど、彼女の弟はそのプレゼントを隠した。

Although the boyfriend had bought a present and put it there, the girlfriend's younger brother hid it away.

Example 3

先生（が/は）答えを教えたのに、弟は低い点数を取った。

My younger brother got a bad grade even though the teacher had told him the answers.

Answer 1

I do feel contrastive sense when は is used in those sentences. That is, with は, each sentence sounds somewhat like "On one hand, A did blah-blah, however B..." and there is a contrast between A's action and B's action. In other words, the subordinate clause will be more emphasized. Both が and は are natural, though.