The "k" and "g" sounds are almost exactly the same in these words; however, the correct way to pronounce them is with a "k" sound.

'K' and 'ch' never make a 'g' sound, at least that I can think of. In those examples, they both make a 'k' sound. However, try saying 'sk' and 'sg' back to back. You'll find the sounds will be indistinguishable if you say the 'sk' quickly. It's still definitely a 'k' sound, but if you say a 'g' no one will notice if you speak at a normal speed.

It's a "k" sound. But it sounds different because it's after an initial. Usually in English this sound is aspirated (more air comes out when you pronounce this sound) like the "k" in Chinese for 看 or 口. But after an /s/ it's not aspirated. That's why it probably sounds more like a /g/, but it's not. I hope this helps :)

Well, is not this just another proof that English spelling is crap? Well, it seems to me that the 'k' sound in school is a little softer than the 'k' in skirt, but there is not much difference. Don't worry, English spelling is confusing.