Stan some trucking schools will have variety and train on each and some wont. Some schools these days only have auto. Find a school that trains with manual.
Where I went to trucking school at (alliance) they had trucks with different transmissions but we were like assigned to the same truck if I remember correctly. I don't know I might be mistaken but not to worry.
Once you learn the principle of shifting the number of gears really won't make all that difference.