Absolutely! If you want to achieve Weaponcraft6, Kraanan 6, and Shal'ille 6, you can work on them together at any stage of the building process. Does it slow the improves or make it harder for each other school? yes, but not so much as you could really tell. I choose to do each school seperately, but that's just me. I don't like going back to things once I have worked on them. I usually get my skills or spells to whatever I can and then move on to the next level (regardless if I can buy the level or not, I get them as high as I can first).

Staggering your schools does have it's benefits. Being able to cast Bless and Super Strength while working on Short Sword Fighting is awesome. Being able to take on Trolls while having Minor heal is great, Holy Weapon works amazing on the undead ... as does Holy Touch. There are many reasons why players choose to stagger schools, I simply choose not to because I like working on them one at a time.

///Arkblade