The only problem with the TI-89 is that it's often banned for some of your freshmen/sophomore level math classes. I think a TI-86 is overkill for high school, but it could carry one all the way through college and beyond, and definitely through the first year or two of college if one wanted to eventually upgrade to the TI-89.
Of course, a TI-83 or 84 could also carry one all the way through college. In fact, you could still get through college using a slide rule. (Not much fun working with matrices, though).
In college, there were probably one or two problems a class where a graphing calculator really came in handy and maybe two or three classes where where a good electronic calculator was nearly indispensable. A spreadsheet or CASS program such as MATLAB works a lot better than a graphing calculator, but, obviously, a student probably isn't going to be able to use their laptop in place of a calculator on tests.
I finished my EE degree just a couple years ago and I used either a Pickett N4-ES Dual Base Hyperbolic Speed Rule or my standard Post 1460 Versalog at least 90% of the time (the N4-ES was better suited for electrical engineering, but the bamboo of the Versalog just had a special feel to it), excel spreadsheets for most of the calculations I did on the computer, Matlab for problems I couldn't set up very well in Excel, and used my TI-86 barely at all in most classes.