Is it mandatory to have all levels of wealth or education in your city for it to be somewhat balanced? That is having $ - $$ - $$ parks and school levels(grade school, high school and university) is necessary for growth. Normally I only go for top tier facilities so I wanted to be sure.
Education... There are two kinds.
Look at the population details screen. This screen will show you how many jobs are available for each wealth level. If you are striving for balance, you'll consult this screen often.
Most buildings have jobs for all three wealth levels. The buildings seem to function fine without all three wealth levels present.
LW - this is the most dense population type. If you want high population numbers (or students), you want more of this type. These are also the main workers in T1 and T2 industry. Low wealth Residential reacts more strongly to unemployment than other wealth types.
MW - Main workers for T3 industry. Half as many people per residential building as LW.
HW - One sixth as many people per residential building as LW. The residential taxes work out to be about the same as other wealth levels when you factor in the parks. Since there are fewer people in those buildings, the traffic and the need for services is greatly reduced. Skip if you need more people in less space.