Learn Portuguese (Portugal): Venho da escola às seis.

It comes from the verb vir (to come). This is a very irregular verb, so its 'I' form (eu venho) looks quite different from the dictionary form. Because the ending makes it clear who is doing the action, the pronoun eu (I) is naturally dropped here.

Why is it da escola instead of just de escola?

In English, we say 'from school' without the word 'the'. But Portuguese usually includes the definite article (the) with institutions like school. So, we combine the preposition de (from) with the feminine article a (the) to make da (from the).

Why does às have an accent mark?

The backward accent (called a grave accent) shows that two words have merged into one. When telling time, you always combine the preposition a (at) with the plural feminine article as (the, referring to the hours). Merging a + as gives you às.

Do I need to add the word for 'hours' after seis?

No, you do not have to. Just like in English where we can say 'at six', in Portuguese you can simply say às seis. You could say às seis horas, but leaving the word for hours out is completely correct and very common.