"Everybody came to the class on time."
Translation:Todos llegaron a la clase a tiempo.
"Todo" is invariable, (not to be confused with "todos", it means the whole, all.' In French, it's the same, "Tout".)
"Todos/Todas" + article, means "every", "all the". "Todos los ciudadanos son libres" = "Every citizen is free". "All the citizens are free". (French: tous, toutes)
Yeah, in everyday language we say "everybody was there" and we may be including ourselves -- perhaps, or not. ;) Here I think the idea is that all students arrived on time (a teacher talking about her class, maybe?), and everybody = "they" => llegaron. llegaron is also the verb form for "you formal, plural" (Ustedes) but I don't think that matters here
Both "todos" and "todo el mundo" mean everyone in Spanish. If you want to use personas, it needs to be "todas personas" so that both words are feminine.
However, I just checked the hints and can see why the hint Todos (personas) might be confusing. (Personas) is in parenthesis to indicate that people are implied but it's not okay to say/write "todos personas." For people (personas), you use todos for all men or a mix of men and women or todas when talking about everyone in a group of women.
Personas is feminine; Todas las personas.
On spanish.stackexchange, they explain:
○ Toda la gente está esperando pacientemente." => "All (of) the people (as a whole) are waiting patiently."
"One last thing to note is that while gente isn't interchangeable with personas, the opposite isn't necessarily true. In cases where emphasis or more formality is desired, personas can be used without specifying a number. Thus in the second example, the sentence could also be written as:
○ "Todas las personas están esperando pacientemente." => "All (of) the (individual) persons are waiting patiently."