I believe all you need to do is sort your result set by date and, depending on whether you sort ascending or descending, use Navigation Script Step Go to Record/Request/Page [Last] (ascending sort) OR Go to Record/Request/Page [First] (descending sort).
I'm just learning FMP15A and the experts may have a better solution.
In the relationship graph, in the Edit Relationship box where you connect Teacher to Teaching History, select to Sort records and have it sort (in descending order) by the Teaching Year (or whatever your field name is)
When referring to related history records from the teacher table, If you place a field from Teaching History directly on the layout without putting it into a portal, you will see data from the first related record. If your relationship sorts on the year field in ascending order, that will be the earliest history record.
Conversely, a calculation field in History can use the Last() function to return data from the last record.
Or you can place a one row portal to Teacher History and sort it by year in descending order.
There are also ways to access these same records using the ExecuteSQL function.