1)All the former classmates are planning on attending the formal reunion ceremony
My book says "we don't plan on doing sth, we plan to do it". But Oxford dictionary says:
plan (on sth / on doing sth) to intend or expect to do sth:
[v] We hadn’t planned on going anywhere this evening. Ç [v to inf] They plan to arrive some time after three. Ç [vn] We’re planning a trip to France in the spring—are you interested?
Stylistically, however, I wouldn't put two -ings so close together, so I'd change are planning to plan.
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I think "planning to attend" is much better than "planning on attending"