Spanish students will often pronounce words starting with
"st", "sp", "sk" as "est..", "esp...", "sk...",
Ask them to sound out the "ssss" sound as in Spanish "si",
then finish with the rest of the word:
For example, for "start":
sssssss-tart
Here is an example sentence:
Stop stepping on my stupid skunk, students,
and start studying and speaking.