Explanation of the sequence of the sounds

The order in which the sounds are introduced does not follow the alphabet, but it is not random.
Sounds and their written correspondence are taught creating a balance between the frecuency of usage of the sound and the amount of words that can be created with all the sounds known so far by the student. 
For example, by the time we have learnt how to pronounce, read and write the sounds s, a, t, p, i, n, we are able to read and write plenty of words such as pin, tip, ant, tap....

sounds around us
goc keurh
green: easiest sounds to write
orange: consonant diagraphs
purple: long vowels
pink: vowel diagraphs