10 minutes longer on the bus / 30 minutes earlier out the door. Crazy or ok?

Does School B start 20 minutes earlier than School A? If so, do they end earlier as well?

Will your DC take the bus home most days as well? If so, what's the difference in when they would get home?

I do think 35 mins or 45 mins on a bus (if that’s the actual bus journey) is a long time. But I agree it is more likely that grammar school DC will be doing this, unless the other school has a very wide catchment too – are you very rural? What kind of walk is there at either end?

If there is (say) a 10-min walk to the bus stop and then a 5-min walk to the school at the other end, for your “nice” nearer school that’s a 50-min commute, longer if time is allowed to wait at the bus stop (tho when I was a teen I frequently raced from a certain point about 300m away when I saw my bus coming round the corner!); for the grammar that’s an hour each way at least. I think that’s a lot but as I say, I am assuming that you live rurally (where I live, in a small city, a commute of an hour on foot and by bus would take you way past <tallies quickly> at least 10 secondary schools).

How far away is your most local school? Why is it not an option? In general I am very much in favour of students attending their local school as long as it is suitable but even then for sure I can think of schools I might live locally to which I would not be super happy about my DC attending.

To answer your actual question (!) my school commute (rural area) was about 45 mins door to door (including the run haha) and IMHO it was too far. I rarely saw school friends outside school which put a big limit to my social life and social skills later I think. But it may be that a longish commute is unavoidable in which case adding 10 mins (if it is only 10 mins) may be acceptable. I would say tho that the fact that lots of others also do a long commute to the grammar (which for sure is likely especially if you are not in a city) is not helpful on the social point. That was also my situation. I had a friend, almost a good friend, who lived 15 miles from school in a different direction from me (I lived about 8 miles away). So 23 miles, with no buses, the other end of the world tbh. But then my parents were not willing to drive me here and there in the evenings and at weekends and I suspect today's parents are more willing to facilitate (I know I was when my DC were younger).

You will likely find that children at the grammar school travel at least that far, some up to an hour/hour and a half as they tend to have quite large catchments (the ones by us certainly do).
if you live in a rural area, 30-45 minutes on a bus is quite normal too, so I am sure both options will be fine for your dc.

A less-than-one-hour journey for a grammar school is a compromise many families are willing to make, whereas a 40 minute commute to an ordinary state school feels much harder to justify.

Is School A an indie, or a faith school? Otherwise, I'm not sure how you can be certain of a place at a school so far away which isn't your local school? (Also presumably your DC has qualified for the grammar school?).

Assuming that both the grammar and School A are actually realistic preferences, wrt the journey I would also consider whether either of the buses is a fixed school bus (one journey am, one home pm), or a public service, since the latter would make it easier to take part in after school activities etc