Well said Marbleman!
I have been tiling for only 26 years so I am still an apprentice. lol
It is quite rare to find situations where you get the perfect job.
Substrates are never flat, levelling compound never self level, 95% of the time the tiles from diy stores are banana shaped, (it must be hard to make them that shape
) screws always stick out, Baths trays always out of level...... I can go for ever.
So, unnless you build the wall and screed the floor yourself and do all fitting and any other prep yourself; what the teach you at school will be of little use. Products and techniques constantly changing so we need to adapt often.
Use comonsense, read the info whenever available, don't cut corners, tell the customer that you need to do more preparation and charge accordingly.