How come sometimes the ku sound does not pronounce?

[hakuma387496]

Why is the ku sound in gakusei does not pronounce but the one in gakuwari does?
Is there a rule to follow? Thanks.

[QuackingShoe]

'u' and 'i' are softened when between unvoiced consonants, such as 'k' and 's'.

[Javizy]

The rule to follow is speak like natives do. Listen to them and mimic them, and you should end up sounding half decent.

[taikutsu]

You know, you could ask this same question about MANY more situations in English. It's like asking why the "ei" in the words "reign" and "ceiling" sound different. Javizy is absolutely correct here about what you should do.

[QuackingShoe]

Be that as it may, this is a pretty simple rule to keep in mind, particularly as it's an incredibly frequent (and consistent) aspect of Japanese phonology.