Each pizza had 8 pieces.

Each pupil took 3 pieces.

3 pizzas had (3 × 8) = 24 pieces and fed (24 pcs ÷ 3 pcs) = 8 pupils

3 pizzas -> 8 pupils

9 pizzas -> 8 × 3 = 24 pupils

Number of pupils in 6B -> 24 + 3 = 27

Thank you very much for help