There's nothing wrong with "If I went to a school in Wales, I would play rugby".
Apparently, the book assumes that the speaker currently goes to a school in Germany, and therefore that the speaker will not play rugby because it's not a readily available option to a school student in Germany. Based on this current state of affairs in the actual world where the speaker resides, the speaker can choose to describe an alternate world where the speaker goes to a school in Wales, where playing rugby is an option readily available to school students.
Since this alternate world does not share space-time with the actual world, the speaker feels remoteness in describing situations in the alternate world, which is reflected in the past form of the verbs went and would.
shouldn't it be: "I would have played rugby"
This form should be used to describe a past situation in the alternate world. But there's no contextual reason to use a more complicated form in the grammar book.