Technically according to the meaning of 不得劲儿(cannot fully utilize your energy; doesn't have enough strength...) It should be 得dé(to obtain). But 不得dei3劲 is more commonly used. It's 方言so that would depend on how you look at "standard". ;)

不得劲儿 has several meanings:

(1).不顺手使不上劲。

这东西用着不得劲儿。

(2).不舒适。

我今天有点不得劲儿不想去上班了。

(3).不好意思。

别这么说他都不得劲儿了。

Here in this sentence 第4首很好听 但是男的唱高音还是有点不得劲

不得劲 is used to describe your student's feeling which is uncomfortable 不舒服, 不合适 - doesn't feel right, not appropriate. So here it's more like mentally 不舒服 or maybe physically - it depends on how your student reacted to that 男高音;)