The 15% test is a Revenue one to determine eligibility for the PAYE employee Tax Credit. The DSFA apply a less scientific approach and take into account the issue of overall control of the enterprise. A shareholder with less than 15% may still be deemed to have control in some circumstances when taken with other connected persons shareholdings and in that case might still be S class.
A <15% shareholder/director with no connection to any of the other shareholder/directors might normally be expected to be on class A, however the only definitive answer is to contact Scope Section DSFA for a ruling.