People enjoying themselves at the beach
..selves. More than one ‘self’. OK, no complaints about that.
..theirselves. Now you’re being inconsistent with your object pronouns.
You guys have no problem producing the following:
And you would never write:
So why the sudden switch to a subject possessive their?! Please use the object pronoun (him, her, them) + ‘self/selves’:
And you might also think about some collocation (prepare + s.o./s.th. + for + s.th.):
Finally, we can assume that the students must prepare themselves and not other people, so strictly speaking themselves is redundant: