11th grade may be the toughest year at any high school, so a BS wants to see that you will be able to meet their academic expectations and adjust to living at a new school - as soon as you arrive.
Many successful 11th grade applicants are athletes who are likely to be recruited in college (so a bobble in academics can be overcome and integration into the community is almost guaranteed through their team) and who have coach support.
When my kid was ar George, there were a number of international students coming in to do the IBD - many from an academic track that was clearly IB compatible.
There is no hard a fast rule on this - it will depend on the school. But your ability to hit the ground running has to be evident in your application, so good grades, preferably backed up by good external validation (maybe some AP scores, etc), will be essential.