Students are sitting lined up on stair steps while they wait and check their electronic devices.
Some students are waiting on a stair case.
Since we know there are students (plural) checking their phones while they wait, it follows that some students are waiting on a stair case.
exists e1 p1 s2 x1 x4.(exists e2 p2 s1 x2 x3.(exists e3 e4.(Theme(e4,x4) & Actor(e4,x1) & v_check(e4) & Actor(e3,x1) & v_wait(e3)) & while(e2,p2) & on(e2,x2) & n_step(x2) & of(x2,x3) & n_stair(x3) & a_up(s1) & Manner(e2,s1) & Theme(e2,x1) & v_line(e2)) & n_device(x4) & a_electronic(s2) & Theme(s2,x4) & of(x4,x1) & n_thing(x1) & n_thing(x1) & Topic(e1,p1) & Actor(e1,x1) & v_sit(e1) & n_student(x1))
exists e1 x1 x2 x3.(on(e1,x2) & n_case(x2) & of(x2,x3) & n_stair(x3) & Actor(e1,x1) & v_wait(e1) & n_student(x1))