How to stop stuttering on words with 'S' sounds.

How to stop stuttering on words with 'S' sounds. I have problem with words beginning with S. Today I was really humiliated by one such incident. It's like when I speak a word which begins with S ,say school. I get a block and very forcefully can get it out as "Hissssschhool". It's really humiliating when the other person is looking right at me. So is there any way to stop getting these blocks on words beginning with S? Any advice is appreciated.