I've been in the industry for several years and I've come across every type of situation. I've met guys from top schools who work infirms, but I've also met people from those same schools who can't seem to land a great job. Conversely, I've met guys from non-target schools who managed to get into a top firm.
Simply stated, I do recommend you attend the best school you can get into. Being the underdog is no fun and even if you "break in" it will take longer to get where you want to go. As well, there are many employers you'll meet along the way that are very rigid about the "rules". So even if you get in, it may still be harder to get promoted or find a better job later on. Therefore, the best solution is to attend the best school. This leads me to the first rule:
Jordan was the best basketball player of his era. Charles Barkley was arguably #2. On any given night, Barkley might outscore Jordan. However, NO ONE would ever remember Charles Barkley as the best player of his era. The same rule applies for b-schools. There are many schools that rank "top 20" and claim they are almost as good as the top b-schools in the country. However, they are the Charles Barkley of b-schools. Even if they are that good... they'll never get you the same amount of respect in the job market.
The next set of rules are suitable for people who don't end up going to target schools:
In similar fashion, if your goal is to do M&A in a firm, you should get a job doing M&A at a boutique or an accounting firm. The same rule applies for lending: a commercial banker will eventually be able to land a corporate lending job. It takes 2-3 years of experience and a bit of patients but you will eventually get in. Also, remember that your time spent doing mid-market IB/FX/lending will not necessarily count as work experience in the big leagues.
Time is on your side. If you are 22 years old and are trying to get into IB, you have 43 years until you reach retirement age (i.e. 65). Even if you end up working in a tier 2 firm from 22 to 25 and then go for an MBA from 25 to 27, you could still end up joining the industry at 28 years old. That still leaves 37 years until retirement! Most successful bankers are considered "old veterans" after 20 years of experience.... With 37 years to go you have nothing to worry about!