Not only with this verb if motion. "prepositions used with verbs of motion to indicate the direction of the motion. Four Russian prepositions govern the accusative case to indicate motion toward a place and either the prepositional or instrumental case to indicate presence at that place. " Source: https://www.alphadictionary.com/rusgrammar/prepacc.html
The site also has a neat little table showing, for the prepositions в, на, за and под, that you ask the following questions:
Куда (where)? => Acc.
Откуда (where from)? => Gen.
Different cases for the question где? are explained too (but I'm not going to summarize that, cos I don't understand it yet).
So, dear fellow users: How do I know whether to use куда or где?
in English 'to go' is differing from 'to walk' is there such a difference in Russian, and does it apply here? In English the sentence could be both: "Are you old enough already for going to school?" but also "It is early in the morning, and you are going to school already!"
Yes we really pronounce these three consonants without any problems. To be honest, I never thought about how difficult it is ) It's hard to me as a person who learned English to get used to pronouncing sounds in a row ending in "th" and "s". It's not possible to quickly pronounce these sounds in a row, they merge into one ))