Complex sentence: But in a twist ending as they reached school Donald was shocked as he noticed that school is closed for summer holidays, which causes the nephews to give him an angry glare, causing Donald to shrink from embarrassment over his mistake.
Split into simpler sentences: But in a twist ending, as they reach school, Donald is shocked when he notices that school is closed for summer holidays. This causes the nephews to give him an angry glare, causing Donald to shrink from embarrassment over his mistake.