You need to explode $schools
into an actual array:
$schools = $_SESSION['sarray'];
$schools_array = explode(",", $schools);
$result = count($schools_array);
if you just need the count, and are 100% sure it's a clean comma separated list, you could also use substr_count() which may be marginally faster and, more importantly, easier on memory with very large sets of data:
$result = substr_count( $_SESSION['sarray'], ",") +1;
// add 1 if list is always a,b,c;
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