Student visa. Course lenghth = 18 months, CoE = 6 months?

[Lomaster]

I've received my Certificate of Eligibility which states "在留資格 留学 ６months" meaning i can only apply for 6 months visa. But the length of my course is one and a half years and that's the minimum duration i intend to stay. My school syas it is a common practice to renew visa after 6 months once already in Japan. Is that really so common? Did anyone have similar experience? Is it a safe bet to sign a one-year apartment rental contract in such case?