Knowing whether to use "c" or "k" confounds many students, but especially students who need structured literacy explicitly taught to them. In order for a dyslexic reader to process the spelling convention rules efficiently, we have to give extremely detailed information on why the language is structured the way it is.
When we were little, many of us heard the rule "k goes with i and e; c takes the other three." This rule can also be applied to the sk or sc blends. Since sk and sc both make the same phonemes, we need to teach what grapheme option is suitable when building a word.
Sk or Sc which beginning blend do I use when spelling? Sk almost always goes with "i" and "e", while sc almost always goes with "a", "o", and "u". Of course, as with most English spelling rules, there are exceptions. The most notable are skate, skull, and skunk. Overall however, this rule rings true about 90% of the time.
This is by no means a complete list of all the sk and sc beginning blends, but it does cover the most used words. I like to print a rule card for my students who need consistent and explicit literacy training on the orthography of the language. Here is a free pdf with two cards if you feel this will be helpful for your students as well. I laminate the cards and put them in the front of their binder.