It's not wrong, but I prefer
El problema es el cuerpo de estudiantes.
El problema es el alumnado/estudiantado.
El problema son los estudiantes.
El costo mayor es el de los trabajadores.
In an informal level, son is OK and es is also OK but it sounds a bit lame (cripple) because it is elevated in the language aspect but not in the exposition of ideas.
By the way, congratulations, as this question has an educated-native speaker level.
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