修学旅行は英語では school trip です。
修学旅行を楽しんで、と言いたいなら
❶ Enjoy your school trip!
(修学旅行楽しんでね！)
❷ Have fun during your school trip!
(修学旅行期間中は楽しんでね！)
〜と言えます。
Tell me all about it when you get back.
(帰って来たら、どんなんだったか教えてね！)
〜参考までに！
Have a great time on your school trip.
Have fun on the school trip.
Have a good time!
Have an awesome time on the trip!
I hope you have a great trip.
Have a great time on your school trip.（修学旅行楽しんできなよ）
Have fun on the school trip.（修学旅行楽しんできなよ）
Have a good time!（楽しんできなよ）
Have an awesome time on the trip!（旅行楽しんできなよ）
I hope you have a great trip.（旅行楽しんできなよ）
When wishing someone well on their trip you could say 'enjoy your trip!' or 'have fun on your trip!'
By adding school you are explaiing what trip they are going on
「旅行を楽しんできてね」は、'enjoy your trip!' あるいは 'have fun on your trip!' と言えます。
'school' を加えて、それが何の旅なのか説明しています。
To wish someone a good trip the first exampled sentence above is probably the most generally apropriate form. The second example is more of a lighthearted comment which is probably meant more as a joke than as concerned advice.
アメリカ合衆国 
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イギリス 
アイルランド 
