When I were a lad...
By the time of 'O' level Maths we were using the old Four Figure Tables for looking up Sines, Tans, Logs etc. One thing it did teach you was an awareness of quantity and sanity checking of your answers - does it sound about the right size or have you slipped a decimal place or two.
For 'A' level in Maths and Physics something a bit more sophisticated was needed so for my 15th birthday my pride and joy was a Thornton slide rule, the double sided model with precision cursor in plastic. I also had an old one of my Dad's dating from when he was at college (Engineering at RNEC Manadon), also a Thornton but made of wood with engraved ivory (I think - might be fake) scales.
I still have them both - and they both still work (no batteries in these babies).
Again the beauty of a slide rule is that you don't get lost in the precision of the answer - four significant figures if you are lucky, and you'll need to intutively know where the decimal point should be. It is a tool to help you find the answer, and using it is a skill which makes a closer connection between you as the calculator and the figures you are manipulating.
The slide rule served well through the first years of Uni on an Engineering course. I went up in Oct 1970 and there was not a calculator in sight. In 1971 we had a 2 week module on the Maths department's computer - another story. By the time I left in 1972 calculators were still desk bound devices with flourescent tube displays and mains powered. There were rummours of seven segment LED digit displays and battery portable calculators but I didn't know anyone who had one.
After another three years on another course where I needed virtually no maths I got a job and bought myself my first pocket calculator - a Sinclair Cambridge Programmable. 8 digit red led display, a nice set of built in functions (some of questionable accuracy), plus the abbility to write simple script programmes and run them. I was ever so proud of my programme to calculate parallel resistor values.
I don't think I have used the slide rules in anger since then - although they do still work.