Service rendered for four-fifths or more of the official school year of the school district or institution in which a teacher is employed shall be credited as a year's service regardless of the length of the school term, but in no case shall more than one year of service be creditable for service rendered in one fiscal year. Service rendered for less than four-fifths of the official school year shall be credited for that portion of the school year for which it was rendered: PROVIDED, That no service of less than twenty days in any school year shall be creditable.