conseguiiii! resolvi mas de forma bem estranha... nao entendi o defeito ainda mas resolveu!
tava assim:
$params = array(5, $idcontext);
$sql = "SELECT DISTINCT u.id, u.firstname,u.lastname FROM {role_assignments} rs INNER JOIN {user} u ON u.id=rs.userid WHERE rs.roleid= ? AND rs.contextid = ?";
$selalunos = $DB->get_records_sql($sql, $params);
mudei para
$selalunos = $DB->get_records_sql('SELECT DISTINCT u.id, u.firstname,u.lastname FROM {role_assignments} rs INNER JOIN {user} u ON u.id=rs.userid WHERE rs.roleid= ? AND rs.contextid = ?', array(5, $idcontext));
rodou na hora!