'pi' requires at least two words after it, so 'tawa pi noka' is wrong as it stands, though 'tawa noka' is fine. Part of the trouble is that 'tawa' is basically a proposition, i.e., should be followed by a noun and 'noka' and 'tomo tawa' are nouns. Here, however, serving as modifiers. When 'tawa' functions as a verb, as in this case, the "to where" is the noun that follows it in its prepositional function. So. if we wanted to do the whole thing using both thse devices, we would end up with 'tawa pi tomo tawa pi mama mije mi, tomo sona pi suli kasi' I put a comma in to mark the break between the modifier to the verb, 'toma tawa pi mama mije mi' and the destination ' tomo sona pi suli kasi' ("school for growing plants"). This las is not the direct object, which is introduced by 'e'. 'pi' does (obviously) not mean "method of locomotion"; it merely separates out right-grouped chunks in a left-grouping structure: so 'tomo tawa' is taken as a unit modifying 'tawa' rather than 'tomo modifying 'tawa' and 'tawa' then modifying 'tawa tomo'
Sorry; that is a lot of tp grammar in a short space. Please ask more questions
The problem with 'tawa tomo' is that it naturally reads as "go to the building" not "go by means of the construction".